Droughts are not uncommon in the State of Oregon, nor are they just an "east of the mountains" phenomenon. They occur in all parts of the state, and in both summer and winter. They appear to be cyclic and they can have a profound effect on the state's economy, particularly the hydro-power and agricultural sectors. The environmental consequences also are far-reaching. They include insect infestations in Oregon forests and the lack of water to support endangered fish species. Severe drought conditions preceded the four disastrous Tillamook fires (1933, 1939, 1945, 1951) and pitted farmer against fish propagation groups during the Klamath Basin drought of 2001. The minimum drought loss included about 1200 jobs and $150 million dollars in goods and services. Local farmers maintain that the cost was considerably more. Water allocation continues to be controversial. In recent years, the state has addressed drought emergencies through the Oregon Drought Council. This interagency (state / federal) council meets to discuss forecasts and advise the Governor as the need arises. Significant Oregon droughts are listed in Table 1.
| DATE | DESCRIPTION |
| 1904-1905 | A statewide drought period of about 18 months |
| 1917-1931 | A very dry period throughout Oregon punctuated by brief wet spells in 1920-21 and 1927 |
| 1939-1941 | A three-year intense drought in Oregon |
| 1959-1964 | Primarily affected eastern Oregon |
| 1985-1997 | Generally a dry period, capped by statewide droughts in 1992 and 1994 |
Oregon's drought history reveals many short-term and a few long-term events. The average recurrence interval for severe droughts in Oregon is somewhere between 8 and 12 years. Table 1 provides an overview of some severe droughts in Oregon.
The probability that Region 5 will experience drought and the region's vulnerability to their effects are depicted in Table 2 below. These scores are based on an analysis of risk conducted by county emergency program managers, usually with the assistance of a team of local public safety officials.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
In some cases, counties either did not rank the hazard or did not find it to be a significant concern. These cases are noted with a dash (-) in the table below.
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | M | - | - | H | - | H |
| Probability | H | - | - | H | - | H |
The geographical position of this region makes it susceptible to earthquakes from four sources: (1) the off-shore Cascadia Fault Zone, (2) deep intra-plate events within the subducting Juan de Fuca plate, (3) shallow crustal events within the North America Plate, and (4) earthquakes associated with renewed volcanic activity. All have some tie to the subducting or diving of the dense, oceanic Juan de Fuca Plate under the lighter, continental North America Plate. Stresses occur because of this movement and there appears to be a link between the subducting plate and the formation of volcanoes some distance inland from the off-shore fault zone.
When crustal faults slip, they can produce earthquakes with magnitudes (M) up to 7.0 and can cause extensive damage, which tends to be localized in the vicinity of the area of slippage. Deep intraplate earthquakes occur at depths between 30 and 100 kilometers below the earth's surface. They occur in the subducting oceanic plate and can approach M7.5. Subduction zone earthquakes pose the greatest hazard. They occur at the boundary between the descending oceanic Juan de Fuca Plate and the overriding North American Plate. This area of contact, which starts off the Oregon coast, is known as the Cascadia Subduction Zone (CSZ). The CSZ could produce an earthquake up to 9.0 or greater.
This part of Oregon has experienced three historic earthquakes of significance that were centered in the region: the 1893 Umatilla (VI or VII Modified Mercalli Intensity), the 1936 Milton-Freewater (M6), 1951 Hermiston, and the 1976 Deschutes Valley (M4.8), all shallow crustal earthquakes. There are also identified faults in the region that have been active in the last 20,000 years. The region has also been shaken historically by crustal and intraplate earthquakes and prehistorically by subduction zone earthquakes centered outside the area (Table 3). Given this history, there is good reason to believe that the most devastating future earthquakes would originate along shallow crustal faults in the region.
Earthquake associated hazards include severe ground shaking, liquefaction of fine-grained soils, and landsliding. The severity of these effects depend on several factors, including the distance from the earthquake source, the ability of soil and rock to conduct seismic energy and the degree (angle) and composition of slope materials.
Earthquakes produced through volcanic activity could reach magnitudes of M5.2. However the Cascade volcanoes are some distance away from populated centers, which tends to lessen the concern.
Earthquake risk in Region 5 is reflected in the Uniform Building Code's (UBC) Earthquake Hazard maps (i.e., seismic zones 1-4). The higher the numerical designation, the more stringent the building standards become. Region 5 is within UBC Seismic Zone 2b.
| DATE | LOCATION | MAGNITUDE (M) | REMARKS |
|---|---|---|---|
| Approximate Years 1400 BCE* 1050 BCE 600 BCE 400 750 900 | Offshore, Cascadia Subduction Zone | Probably 8-9 | Based on studies of earthquake and tsunamis at Willapa Bay, Washington. These are the midpoints of the age ranges for these six events. |
| January, 1700 | Offshore, Cascadia Subduction Zone | Approximately 9.0 | Generated a tsunami that struck Oregon, Washington, and Japan; destroyed Native American villages along the coast |
| March, 1893 | Umatilla | VI-VII (Modified Mercalli Intensity) | Damage unknown |
| July, 1936 | Milton-Freewater | 6.1 | Eastern Oregon's largest event, several aftershocks, $100, 000 dollars in damage based on 1936 dollars, chimney damage, houses shifted off foundations, school buildings damaged |
| January, 1951 | Hermiston | V | Damage unknown |
| April, 1976 | Deschutes Valley | 4.8 | Near Maupin, cracked plaster, objects thrown |
Notes: * BCE: Before the Common Era
Source: Ivan Wong and Jacqueline D.J. Bolt, November 1995, A Look Back at Oregon's Earthquake
History, 1841-1994, Oregon Geology, pp. 125-139.
The Cascadia Subduction Zone generates an earthquake on average every 500-600 years. However, as with any natural process, the average time between events can be misleading. Some of the earthquakes may have been 150 years apart with some closer to 1,000 years apart (DOGAMI, 1999). Establishing a probability for crustal earthquakes is more difficult given the paucity of historic events in the region. Earthquakes generated by volcanic activity in Oregon's Cascade Range are possible, but likewise unpredictable.
Region 5 is moderately vulnerable to earthquake hazards from earthquake-induced landslides in the Cascades and ground shaking.
The Oregon Department of Geology and Mineral Industries (DOGAMI) has developed two earthquake loss models for Oregon based on the two most likely sources of seismic events: (1) the Cascadia Subduction Zone (CSZ), and (2) combined crustal events (500-year Model). Both models are based on HAZUS, a computerized program, currently used by the Federal Emergency Management Agency (FEMA) as a means of determining potential losses from earthquakes. The CSZ event is based on a potential 8.5 earthquake generated off the Oregon coast. The model does not take into account a tsunami, which probably would develop from the event. The 500-Year crustal model does not look at a single earthquake (as in the CSZ model); it encompasses many faults, each with a 10% chance of producing an earthquake in the next 50 years. The model assumes that each fault will produce a single "average" earthquake during this time. Neither model takes unreinforced masonry buildings into consideration
DOGAMI investigators caution that the models contain a high degree of uncertainty and should be used only for general planning purposes. Despite their limitations, the models do provide some approximate estimates of damage. Results are found in Tables 4 to 6.
| REGION 5 COUNTIES | ECONOMIC BASE IN THOUSANDS (1999) | GREATEST ABSOLUTE LOSS IN THOUSANDS (1999) FROM A M 8.5 CSZ EVENT | GREATEST ABSOLUTE LOSS IN THOUSANDS (1999) FROM A 500-YEAR EVENT |
|---|---|---|---|
| Gilliam | $112,000 | Less than $1,000 | $1,000 |
| Hood River | $1,029,000 | $3,000 | $62,000 |
| Morrow | $365,000 | Less than $1,000 | $10,000 |
| Sherman | $97,000 | Less than $1,000 | $1,000 |
| Umatilla | $2,998,000 | Less than $1,000 | $68,000 |
| Wasco | $1,260,000 | Less than $1,000 | $25,000 |
| REGION 5 COUNTIES: | Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | REMARKS |
|---|---|---|---|---|---|---|---|
| INJURIES | 0 | 0 | 0 | 0 | 0 | 0 | These figures have a high degree of uncertainty and should be used only for general planning purposes. The HAZUS run that produced these figures did not account for unreinforced masonry structures. |
| DEATHS | 0 | 0 | 0 | 0 | 0 | 0 | |
| DISPLACED HOUSEHOLDS | 0 | 0 | 0 | 0 | 0 | 0 | |
| ECONOMIC LOSSES FOR BUILDINGS | $5,000 | $3 million | $97,000 | $17,000 | $236,000 | $795,000 | |
| OPERATIONAL THE DAY | |||||||
| AFTER THE EVENT | |||||||
| Fire stations | 100% | 99% | 100% | 100% | 100% | 99% | |
| Police stations | 100% | 100% | 100% | 100% | 100% | 100% | |
| Schools | 100% | 98% | 100% | 100% | 100% | 100% | |
| Bridges | 100% | 95% | 100% | 99% | 100% | 99% | |
| ECONOMIC LOSSES TO | |||||||
| INFRASTURCTURE | 0 | $704,000 | 0 | $29,000 | 0 | $71,000 | |
| Highways | 0 | $76,000 | 0 | 0 | 0 | 0 | |
| Airports | 0 | $17,000 | 0 | 0 | 0 | $6,000 | |
| Communications | |||||||
| DEBRIS GENERATED (thousands of tons) | 0 | 1 | 0 | 0 | 0 | 1 |
| REGION 5 COUNTIES | Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | REMARKS |
|---|---|---|---|---|---|---|---|
| INJURIES | 0 | 30 | 3 | 0 | 19 | 6 | NA* : Because the 500-year model includes several |
| DEATHS | 0 | 1 | 0 | 0 | 0 | 0 | |
| DISPLACED | 0 | 56 | 10 | 0 | 81 | 23 | |
| HOUSEHOLDS | earthquakes, the | ||||||
| number of facilities operational the | |||||||
| ECONOMIC LOSSES FOR BUILDINGS | $705,000 | $62 million | $10 million | $923,000 | $67,000 | $25 million | |
| OPERATIONAL THE | day after the | ||||||
| DAY AFTER THE | quake can not be | ||||||
|
EVENT Fire stations Police stations Schools Bridges |
N/A* N/A* N/A* N/A* |
N/A* N/A* N/A* N/A* |
N/A* N/A* N/A* N/A* |
N/A* N/A* N/A* N/A* |
N/A* N/A* N/A* N/A* |
N/A* N/A* N/A* N/A* |
calculated. The HAZUS run that produced these figures did not account for unreinforced masonry structures. |
|
ECONOMIC LOSSES TO INFRASTRUCTURE Highways Airports Communications |
$350,000 $440,000 $29,000 |
$12M $3M $1M |
$550,000 $392,000 $46,000 |
$3 million $423,000 $61,000 |
$6 million $3 million $3 million |
$3 million $2 million $1 million |
|
| Debris generated (thousands of tons) | 0 | 41 | 8 | 0 | 45 | 16 |
The probability that Region 5 will experience earthquakes and the region's vulnerability to their effects are depicted in Table 7 below. These scores are based on an analysis of risk conducted by county emergency program managers, usually with the assistance of a team of local public safety officials.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | L | M | H | L | M | M |
| Probability | H | M | L | L | M | L |
Oregon has a very lengthy history of fire in the undeveloped wildlands and in the developing urban/wildland interface. In recent years, the cost of fire suppression has risen dramatically; a large number of homes have been threatened or burned, more fire fighters have been placed at risk, and fire protection in wildland areas has been reduced. These factors have prompted the passage of Oregon Senate Bill (SB) 360 (Forestland / Urban Interface Protection Act, 1997). This bill: (1) establishes legislative policy for fire protection, (2) defines urban/wildland interface areas for regulatory purposes, (3) establishes standards for locating homes in the urban/wildland interface, and (4) provides a means for establishing an integrated fire protection system.
This document defines wildfire as an uncontrolled burning of forest, brush, or grassland. Wildfire always has been a part of these ecosystems and sometimes with devastating effects. Table 8 provides an overview of the significant wildfires Oregon, an important indicator of the type of fires possible in the region. Wildfire results from natural causes (e.g., lightening strikes), a mechanical failure (Oxbow Fire), or human-caused (unattended campfire, debris burning, or arson). The severe fire season of 1987 resulted in a record setting mobilization of fire fighting resources. Most wildfires can be linked to human carelessness.
Region 5 contains a variety of forest and grassland ecosystems. The Cascade Mountains form the western boundaries of Hood River and Wasco counties. Morrow and Umatilla counties contain large tracts of Blue Mountain forests and all Region 5 counties have extensive grasslands. Each ecosystem is different. Consequently, the probability and management of wildfire would differ from place to place. The build- up of fuel (e.g., brush, dead or dying trees) that leads to devastating wildfires is a very important factor and is the current focus of mitigation strategies.
| Year | Name of Fire | Location | Acres Burned | Remarks |
|---|---|---|---|---|
| 1977 | Wasco | |||
| 1979 | Pine Grove/Juniper Flat | |||
| 1983 | Moro | Sherman | ||
| 1985 | Maupin | Wasco | ||
| 1988 | Wasco | |||
| 1991 | Falls | 1,100 | Fire along the Columbia Gorge. | |
| 1994 | Smith Canyon | |||
| 1998 | Rowena | Wasco | 2,208 | |
| 1998 | Reith Barnhart/Coombs Canyon | Umatilla | 45,000 | |
| 2000 | Willow Creek | Morrow and Gilliam | 27,000 | |
| 2000 | Antelope | Wasco | ||
| 2001 | Two Rivers | Umatilla | 7,011 | |
| 2001 | Bridge Creek | Umatilla | 9,230 | |
| 2002 | Sheldon Ridge | Wasco | 12,681 |
The probability of a wildland urban interface fire occurrence in this region has been assessed at the local level; each of the counties in this region considers the likelihood of an event to be high.
An understanding of risk begins with the knowledge that wildfire is a natural part of forest and grassland ecosystems. Past forest practices included the suppression of all forest and grassland fires. This practice, coupled with hundreds of acres of dry brush or trees weakened or killed through insect infestation, has fostered a dangerous situation. Present state and national forest practices include the reduction of understory vegetation through thinning and prescribed (controlled) burning.
Each year a significant number of people build homes within or on the edge of the forest (urban/wildland interface), thereby increasing wildfire hazards. In Oregon, there are about 240,000 homes worth around $6.5 billion within the urban/wildland interface. Such development has greatly complicated firefighting efforts and significantly increased the cost of fire suppression. Interface communities at risk in Region 5 are listed in Table 9. A number of these communities are grassland communities rather than forest.
A detailed community inventory of factors that affect vulnerability is important in assessing risk and is beyond the scope of the statewide assessment.
When assessing the risks from natural hazards, established mitigation practices already provide benefits in reduced disaster losses. It is important for communities to understand the benefits of past mitigation practices when assessing their risks, being mindful of opportunities to further reduce losses.
Possible mitigation practices include:
| GILLAM COUNTY | HOOD RIVER COUNTY | MORROW COUNTY | SHERMAN COUNTY | UMATILLA COUNTY | WASCO COUNTY |
|---|---|---|---|---|---|
| Arlington | Cascade Locks | Blake's Addition | Biggs Junction | Gibbon | Antelope |
| Condon | Dee | Boardman | Grass Valley | Hermiston | Bear Springs |
| Mayville | Hood River | Cutsforth Park | Kent | Lehman Springs | Big Muddy Ranch |
| Mt. Hood | Hardman | Moro | McNary | Boyd | |
| Oak Grove | Heppner | Rufus | Meacham | Chenoweth | |
| Odell | Ione | Wasco | Meacham Lake | Cherry Heights | |
| Parkdale | Irrigon | Mill Creek | Clarno | ||
| Pine Grove | Lexington | Milton-Freewater | Durur | ||
| Rockford | Pentland Lake | Mission | Kahneeta Hot Springs | ||
| Summit | Pendleton | Maupin | |||
| Trout Creek | Pilot Rock | Mosier /7 Mill Hill | |||
| Viento | Poverty Flats | North Junction | |||
| Westside | Power City | Oak Springs | |||
| Wyeth | Rieth | Pine Grove | |||
| Stanfield | Rowena | ||||
| Thorn Hollow | Shaniko | ||||
| Tollgate | Sidwalter | ||||
| Ukiah | Simnasho | ||||
| Umatilla | Taylorville/Sportsmans Park | ||||
| Weston | The Dalles/Mill Cr/7 Mile Hill | ||||
| Weston Mountain | Tygh Valley | ||||
| Wamic/ Pine Hollow / | |||||
| Wapintia |
The probability that Region 5 will experience interface fires and the region's vulnerability to their effects are depicted in Table 10 below. These scores are based on an analysis of risk conducted by county emergency program managers, usually with the assistance of a team of local public safety officials.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | L | M | M | H | M | H |
| Probability | H | H | H | H | H | H |
The Mid-Columbia region of Oregon is subject to a variety of flood conditions. The most common type of flooding is associated with unseasonably warm weather during the winter months, which quickly melts high-elevation snow. This condition has produced devastating floods throughout the region (Table 11). The warm weather events usually occur December through February, and can affect the entire state. Flash floods are almost always a summer phenomenon and are associated with intense local thunderstorms. The flash flood of June 1903 in the City of Heppner (Morrow County) is a benchmark event. No flood in Oregon has been more lethal: 247 fatalities. Heppner's vulnerability to flash flood hazards has since been reduced through the construction of the Willow Creek Dam. The region's other flood events are linked to normal seasonal snowmelt and run-off from agricultural fields.
There are several rivers in the region that produce extreme flood conditions. Surprisingly, the Columbia is not one of them, nor is the lower Deschutes or the John Day. The Columbia is so regulated by up- stream dams that it does not present much of a problem. This is partly reflected in the federal flood insurance rate maps for the various communities along the river. However, a swollen Columbia can back up tributary streams to the point where they constitute a significant hazard. This has occurred on a number of occasions. The lower Deschutes and John Day (Columbia River tributaries) are confined to fairly deep canyons with small floodplains. Consequently, they do not present the flood problems associated with smaller rivers, such as the Umatilla, the Walla Walla, and their tributaries. Table 12 details the rivers causing principle flood hazards in the region.
| DATE | LOCATION | DESCRIPTION | TYPE OF FLOOD |
|---|---|---|---|
| June, 1894 | Main stem Columbia River (Region 5 communities) | Largest flood observed on the Columbia River (1,200,000 cfs). City of Umatilla inundated. Widespread damage. | Snow melt (SM) |
| June, 1903 | Willow Creek (Morrow County) | Very devastating flash flood. Forty-foot wall of water in City of Heppner. 247 Fatalities; 141 homes destroyed. | Flash flood (FF) |
| Jan., 1923 | Mid-Columbia region | Widespread flooding. Unusually warm weather, intense rain. | Rain-onsnow (ROS) |
| Jan., 1933 | Mid-Columbia region | Widespread flooding. Heavy mountain snow pack followed by rain and mild temperatures. | ROS |
| Dec., 1955 | Mid-Columbia region | Mild temperatures and rain. Farms, highways flooded. | ROS |
| Dec., 1964 | Entire State | Record-breaking floods throughout state. Heavy snow in mountains followed by intense rain. Considerable flood damage | ROS |
| July, 1965 | Lane / Spears Canyons (Umatilla Co.) | Thunderstorm. Eight to ten-foot wall of water from canyon. Considerable damage. One fatality; several people injured | FF |
| Dec., 1980 | Polallie Creek (Hood River Co.) | Debris flow from vicinity of Mt. Hood. Debris dam formed a small lake that was later breeched. Damage to highways and utilities. | Debris flow |
| Feb., 1985 | Umatilla County | Warm rain on snow at higher elevations. Flooding throughout county. | ROS |
| Feb., 1986 | Entire state | Warm rain on snow. Widespread flooding. Considerable damage | ROS |
| May, 1998 | Central and eastern Oregon | Widespread flooding. Rain melting mountain snow. | ROS |
| Gilliam County | Hood River County | Morrow County | Sherman County | Umatilla County | Wasco County |
|---|---|---|---|---|---|
| Columbia River* | Columbia River* | Columbia River* | Columbia River* | Columbia River* | Columbia River* |
| Thirty Mile Creek | Hood River | Hinton Creek | Birch Creek | Spanish Hollow Creek | |
| Indian Creek | Little Blackhorse Canyon Cr. | McKay Creek | Fifteen Mile Creek | ||
| Shobe Creek | Mill Creek | Mosier Creek | |||
| Willow Creek | Patawa Creek | ||||
| Rhea Creek | Stage Gulch | ||||
| Tutuilla Creek | |||||
| Umatilla River | |||||
| Walla Walla River | |||||
| Waterman Gulch | |||||
| Pine Creek | |||||
| Greasewood Creek |
The probability of an occurrence has been assessed at the county level. Each of the counties in this region considers the probability to be either high or medium. More information follows below.
The probability that Region 5 will experience flooding and the region's vulnerability to their effects are depicted in Table 13 below. These scores are based on the perceptions of area emergency managers.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | L | M | H | H | M | M |
| Probability | M | M | H | H | H | H |
Landslides include any detached mass of soil, rock, or debris that moves down a slope or stream channel. They are classified according to the type and rate of movement and the kind of material that is transported. Debris flows (mudslides, mudflows, debris avalanches) are a common type of rapidly moving landslide that generally occur during intense rainfall on previously saturated ground. They usually begin on steep hillsides as slumps or slides that liquefy, accelerate to speeds as great as 35 mph or more, and flow down slopes and channels onto gently sloping ground. Their consistency ranges from watery mud to thick, rocky, mud-like wet cement --- dense enough to carry boulders, trees, and automobiles. Debris flows from different sources can combine in canyons and channels, where their destructive power is greatly increased. In general, slopes over 25%, or having a history of landslides, signal a potential problem. Landslides / debris flows occur throughout Region 5, but especially in the Columbia River Gorge (i.e., Hood River and Wasco counties).
The Columbia River Gorge is known for its landslide topography, and many of the landslides are very ancient. Landslide / debris flow conditions are worsened by the same weather conditions that produce severe flooding throughout Oregon: rain-on-snow. In short, it is not uncommon in the Pacific Northwest for mild rainy conditions to follow an abundant snowfall. Such was the case in February 1996, when similar weather conditions produced over 700 landslides/ debris flows throughout the state. During that period three landslides closed Interstate Highway 84 along the Columbia River for a period of time. The weather pattern appears to be cyclic.
Landslides / debris flows in Oregon were particularly noteworthy in 1964, 1982, 1966, 1996, and 1997. Research undertaken by the Oregon Department of Forestry has linked many of these landslides to weather and forest management practices (e.g., roads and harvesting); other research efforts have associated landslides with soil types (e.g., loess in the Blue Mountain region or marine sediments in the Columbia River Gorge) and underlying structure (i.e., type and attitude of rocks, etc.). No doubt all of these things are factors. The most universal link, however, appears to be precipitation, which is the basis of Oregon's debris flow warning system.
Oregon's landslide / debris flow warning system primarily involves three state and one federal agency: the Oregon Department of Forestry (ODF), the Oregon Department of Geology and Mineral Industries (DOGAMI), the Oregon Department of Transportation (ODOT), and the National Oceanic and Atmospheric Administration (NOAA). The warning system is triggered by rainfall and monitored in areas that have been determined to be hazardous.
As the lead agency, ODF is responsible for forecasting and measuring rainfall from storms that may trigger debris flows. Advisories and warnings are issued as appropriate. Information is broadcast over NOAA weather radio and on the Law Enforcement Data System. DOGAMI provides additional information on debris flows to the media; ODOT provides information concerning the location of landslides / debris flows, alternate transportation routes, etc.
The probability of rapidly moving landslide occurring depends on a number of factors; these include steepness of slope, slope materials, local geology, vegetative cover, human activity, and water. There is a strong correlation between intensive winter rainstorms and the occurrence of rapidly moving landslides (debris flows); consequently, the Oregon Department of Forestry tracks storms during the rainy season, monitors rain gages and snow melt, and issues warnings as conditions warrant. Given the correlation between precipitation / snow melt and rapidly moving landslides, it would be feasible to construct a probability curve. The installation of slope indicators or the use of more advanced measuring techniques could provide information on slower moving slides.
Geo-engineers with the Oregon Department of Forestry estimate widespread activity about every 20 years; In western Oregon, landslides at a local level can be expected every 2 or 3 years.2
The probability that Region 5 will experience landslides and the region's vulnerability to their effects are depicted in Table 14 below. These scores are based on an analysis of risk conducted by county emergency program managers, usually with the assistance of a team of local public safety officials.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
In some cases, counties either did not rank the hazard or did not find it to be a significant concern. These cases are noted with a dash (-) in the table below.
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | - | - | M | - | - | L |
| Probability | - | - | H | - | - | L |
The western boundary of Hood River and Wasco counties coincide with the Cascade Range. Several of their communities are very close to Mt. Hood, a well-known volcanic peak. In addition, both counties are less than 100 miles from Mt. St. Helens and Mt. Adams in Washington State, two prominent volcanoes. The principal risks from these mountains include air borne tephra (ash), lahars, and pyroclastic flows from a Mt. Hood eruption. The primary risks from Mt. St. Helens and Mt. Adams, separated by distance and the Columbia River, include air borne tephra and the possibility of lahars reaching the Columbia River from Mt. Adams. The remaining counties in Region 5 are at risk from air borne tephra from several Cascade volcanoes. The history of volcanic activity in the Cascade Range is contained in its geologic record; the age of the volcanoes vary considerably. Some lava flows on Washington's Mt. Rainier are thought to be older than 840,000 years; Mt. Saint Helens erupted in May 1980, and continues to be active. In short, all of the Cascade volcanoes are characterized by long periods of quiescence and intermittent activity. And these characteristics make predictions, recurrence intervals, or probability very difficult to attain. Probability
Mt. St. Helens remains a probable source of air borne tephra. It has repeatedly produced voluminous amounts of this material and has erupted much more frequently in recent geologic time than any other Cascade volcano. It blanketed Yakima and Spokane, Washington during the 1980 eruption and it continues to be a concern. The location, size and shape of the area affected by tephra fall are determined by the vigor, and duration of the eruption and the wind direction. Because wind direction and velocity vary with both time and altitude, it is impossible to predict the direction and speed of tephra transport more than a few hours in advance.3
Mt. Hood's eruptive history can be traced to late Pleistocene times (15- 30,000 years ago) and will no doubt continue. But the central question remains: When? The most recent series of events (1900-2000) consisted of small lahars and debris avalanches; Steam explosions and minor tephra falls occurred between 1856 and 1865. Mt. Hood's recent history also includes tephra falls, dome building, lahars, pyroclastic flows and steam explosions. These occurred about 200 years ago. Geoscientists have provided some estimates of future activity in the vicinity of Crater Rock, a well-known feature on Mt. Hood. They estimate a 1 in 300 chance that some dome activity will take place in a 30-year period (1996-20026). For comparison, the 30-year probability of a house being damaged by fire in the United States is about 1 in 90.4
The probability of 1 cm or more of tephra fall-out from eruptions anywhere in the Cascade Range, include:
The probability that Region 5 will experience volcano-related hazards and the region's vulnerability to them are depicted in Table 15 below. These scores are based on an analysis of risk conducted by county emergency program managers, usually with the assistance of a team of local public safety officials.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
In some cases, counties either did not rank the hazard or did not find it to be a significant concern. These cases are noted with a dash (-) in the table below.
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | - | M | - | L | - | H |
| Probability | - | L | - | L | - | M |
Extreme winds are experienced in all of Oregon's eight regions. The most persistent high winds occur along the Oregon Coast and the Columbia River Gorge, so much so that these areas have special building code standards. All manufactured homes in Region 5 that are within 30 miles of the Columbia River, must meet special anchoring (i.e., tie-down) standards (Section 307: Wind Resistance). High winds in this area of Oregon are legendary. The Columbia Gorge is the most significant east-west gap in the mountains between California and Canada. It serves as a funnel for east and west winds, where direction depends solely on the pressure gradient. Once set in motion, the winds can attain speeds of 80 mph, halt truck traffic, and damage a variety of structures and facilities. The average wind speed at Hood River is 13 mph, not much less than the notoriously windy Texas and Kansas plains whose wind speeds average 15 mph.6
A historic overview of windstorms affecting Region 5 is listed in Table 16.
Though their occurrence is somewhat less frequent, Region 5 has also experienced tornadoes. For the most part, these tornadoes have not resulted in major damages. Table 17, below, describes the history of tornadoes in the region.
| DATE | AFFECTED AREA | CHARACTERISTICS |
|---|---|---|
| Apr., 1931 | N. Central Oregon | Unofficial wind speeds reported at 78 mph. Damage to fruit orchards and timber. |
| Dec., 1935 | W. Columbia Gorge | Damage to automobiles. Wind gusts at 120 mph |
| Nov. 1011, 1951 | Statewide | Widespread damage; transmission and utility lines; Wind speed 40-60 mph; Gusts 75-80 mph |
| Dec., 1951 | Statewide | Wind speed 60 mph in Willamette Valley. 75 mph gusts. Damage to buildings and utility lines. |
| Dec., 1955 | Statewide | Wind speeds 55-65 mph with 69 mph gusts. Considerable damage to buildings and utility lines |
| Nov., 1958 | Statewide | Wind speeds at 51 mph with 71 mph gusts. Every major highway blocked by fallen trees |
| Oct., 1962 | Statewide | Columbus Day Storm; Oregon's most destructive storm to date. 116 mph winds in Willamette Valley. Estimated 84 houses destroyed, with 5,000 severely damaged. Total damage estimated at $170 million |
| Mar., 1971 | Most of Oregon | Greatest damage in Willamette Valley. Homes and power lines destroyed by falling trees. Destruction to timber in Lane Co. |
| Nov., 1981 | Statewide | Severe wind storm |
| Dec., 1987 | Umatilla County | Damaging wind storm; 2 fatalities |
| Mar., 1991 | Mid – Columbia / NE Oregon | Severe wind storm |
| Dec., 1991 | N. Central Oregon | Severe wind storm; Blowing dust. |
| Jan., 1993 | Northern Oregon | Severe wind storm. Damage to utilities |
| Dec., 1995 | Statewide | Severe wind storm. Widespread Damage |
| DATE | LOCATION | RESULT |
|---|---|---|
| June, 1888 | Morrow County (Lexington, Sand Hill, Pine City) | 30 buildings, including two schools destroyed. Six people killed (including two children); 4 people injured |
| April , 1925 | Gilliam County | Warehouse and automobiles destroyed in Condon. About $10,000 in damages |
| April , 1957 | Gilliam and Morrow Counties | Minor damage (rangeland) |
| April, 1970 | Wasco County | Observed. No damage |
| May, 1991 | Umatilla County | Some damage to wheat fields |
| July, 1995 | Umatilla County | Some damage to wheat fields |
The probability of an occurrence has been assessed at the county level. Each of the counties in this region considers the probability for future windstorms to be either high or medium. More information follows below.
Many buildings, utilities, and transportation systems within Region 5 are vulnerable to wind damage. This is especially true in open areas, such as natural grasslands or farmlands. It also is true in forested areas, along tree-lined roads and electrical transmission lines, and on residential parcels where trees have been planted or left for aesthetic purposes. Structures most vulnerable to high winds include insufficiently anchored manufactured homes and older buildings in need of roof repair. The Oregon Department of Administrative Service's inventory of state-owned and operated buildings includes an assessment of roof conditions as well as the overall condition of the structure. Oregon Emergency Management has arranged this information by county.
Fallen trees are especially troublesome. They can block roads and rails for long periods, which can affect emergency operations. In addition, up-rooted or shattered trees can down power and/or utility lines and effectively bring local economic activity and other essential facilities to a standstill. Much of the problem may be attributed to a shallow or weakened root system in saturated ground. Uprooted trees growing next to a house have destroyed roofs when they fall as a result of windstorms. In some situations, strategic pruning may be the answer. Prudent counties will work with utility companies in identifying problem areas and establishing a tree maintenance and removal program.
The probability that Region 5 will experience windstorms and the region's vulnerability to their effects are depicted in Table 18 below. These scores are based on an analysis of risk conducted by county emergency program managers, usually with the assistance of a team of local public safety officials.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
In some cases, counties either did not rank the hazard or did not find it to be a significant concern. These cases are noted with a dash (-) in the table below.
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | - | H | H | M | H | M |
| Probability | - | H | M | H | H | M |
Within the State of Oregon, Region 5 communities are known for cold winter conditions. This is advantageous in at least one respect: in general, the region is prepared, and those visiting the region during the winter usually come prepared. However, there are occasions when preparation cannot meet the challenge.
Drifting, blowing snow has brought highway traffic to a standstill. Also, windy and icy conditions have closed Oregon's principal east-west transportation route, Interstate Highway 84, for hours. In these situations, travelers must seek accommodations --- sometimes in communities where lodging is very limited. And local residents also experience problems. During the winter, heat, food, and the care of livestock are everyday concerns. Access to farms and ranches can be extremely difficult and present a serious challenge to local emergency managers. Table 19 provides an historic overview of severe winter conditions within Region 5.
The recurrence interval for severe winter storms throughout Oregon is about every 13 years, however, there can be many localized storms between these periods.
| DATE | LOCATION | REMARKS |
|---|---|---|
| Dec., 1861 | Entire state | Storm produced between 1 and 3 feet of snow throughout Oregon |
| Dec., 1884 | Columbia Basin | Heavy snowfall. The Dalles received 29.5 inches in one day. |
| Dec., 1885 | Wasco County | Most snow ever recorded (6-10 feet). Trains had difficulty reaching Portland. |
| Dec., 1892 | Northern counties | Between 15 and 30 inches of snow fell throughout the northern counties |
| Jan., 1916 | Entire state | Two storms. Very heavy snowfall, especially in mountainous areas |
| Jan., Feb., 1937 | Entire state | Deep snow drifts |
| Jan., 1950 | Entire state | Record snow falls; Property damage throughout state. |
| Mar., 1960 | Entire state | Many automobile accidents; Two fatalities |
| Jan., 1969 | Entire state | Heavy snow throughout state |
| Jan., 1980 | Entire State | Series of string storms across state. Many injuries and power outages. |
| Feb., 1985 | Entire state | Two feet of snow in northeast mountains; Downed power lines. Fatalities |
| Feb., 1986 | Central / Eastern Oregon | Heavy snow in Deschutes Basin. Traffic accidents; Broken power lines |
| Mar., 1988 | Entire state | Strong winds; Heavy snow |
| Feb., 1990 | Entire state | Heavy snow throughout state |
| Nov., 1993 | Cascade Mountains | Heavy snow throughout region |
| Mar., 1994 | Cascade Mountains | Heavy snow throughout region |
| Winter 1998-99 | Entire state | One of the snowiest winters in Oregon history (Snowfall at Crater Lake: 586 inches) |
The probability that Region 5 will experience winterstorms and the region's vulnerability to their effects are depicted in Table 20 below. These scores are based on an analysis of risk conducted by county emergency program managers, usually with the assistance of a team of local public safety officials.
The probability scores below address the likelihood of a future major emergency or disaster within a specific period of time, as follows:
High = One incident likely within a 10 to 35 year period.
Moderate = One incident likely within a 35 to 75 year period.
Low = One incident likely within a 75 to 100 year period.
The vulnerability scores address the percentage of population or region assets likely to be affected by a major emergency or disaster, as follows:
High = More than 10% affected
Moderate = 1-10% affected
Low = Less than 1% affected
| Gilliam | Hood River | Morrow | Sherman | Umatilla | Wasco | |
| Vulnerability | H | H | H | M | H | H |
| Probability | H | H | H | M | H | H |