Mill Creek Floods:  History and Hazards
by Charlie Woodruff
 

Introduction

Mill Creek has had a long history of flooding that has greatly affected the people who live along it.  Shortly after Marcus and Narcissa Whitman settled in the Walla Walla Valley in 1836, other settlers began to move into the area.  Because it was the most suitable place for these people to build houses, the floodplain of Mill Creek and the Walla Walla River became heavily developed.  Walla Walla became a city in 1859, and since the time when extensive structures began to be built on the floodplain, the dangers associated with flooding have increased. The result has been that more structures and lives are at risk.  Although engineering projects and building codes have attempted to minimize flooding risks and damage, residents along Mill Creek need to be aware of the inherent danger of living on a floodplain.

Background

Upper Mill Creek is situated several miles east of Walla Walla in the Blue Mountains.  Due to its great beauty, proximity to Walla Walla, diversity of wildlife, and relative seclusion from the urban center, the upper Mill Creek area (Kooskooskie) has attracted a great number of people to live in its narrow canyon.

Because of the canyon’s narrow and steep sided nature, people are forced to live in homes situated on or very near the flood plain (the floodplain takes up most of the valley floor).  The structures located directly adjacent to the creek are susceptible to even the smallest flooding events. Great hazards are associated with flooding in the Kooskooskie area, which is compounded by the nature of the floods that occur.

This cool, quiet, peaceful canyon is quickly transformed in the event of a flood.  Roads can be washed out, trapping residents in the canyon.  Houses can be destroyed by rushing waters or buried by debris flows.  Thousands of dollars in damage can occur in only minutes or hours.

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Geology

The geology of the Kooskooskie area includes the Columbia River basalts, lava flows about 15 million years old.  These flows were fed by basaltic dikes formed from deep cracks in the crust of the earth caused by regional extension.  Mill Creek has cut down through the basalt layers and formed a steep sided canyon with a relatively narrow valley bottom.  The sides of the canyon consist partly of basalt cliffs, but mostly of colluvium, which is unconsolidated soil and rocks that have moved down the slope.

Upper Mill Creek drops 72 ft per mile compared to lower Mill Creek, which drops only 40 feet per mile (CFHMP 4-10).  The steepness of the slope of the creek combined with the narrowness of the canyon, causes floods to be deep and have high velocity.  Floods in Mill Creek have a relatively short lag time, meaning that the peak discharge occurs soon after the peak of a precipitation event.  These characteristics classify the stream as a “flashy” stream, which poses greater flooding risk than that of a non-flashy stream.  Together with the fact that many houses are built directly along the edge of the creek, these factors result in high amounts of flood damage.

During flood events Mill Creek has a high sediment load.  The load of the stream during a flood consists of clays, silts, sands, pebbles, cobbles and boulders.  The steep sides of the canyon tend to be easily eroded during a major precipitation event, especially if the slopes have been destabilized by grazing or logging practices.  The high sediment load during a flood increases the power of the stream, which, in turn, increases its erosive capability, explaining why such great amounts of damage can occur during floods.


Landslides in the Blue Mountains after the flood of 1996

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Land Use and Related Hazards

The land in the bottom of the canyon along upper Mill Creek is primarily used for residences.  Small amounts of land are used for grazing livestock, logging and agricultural plots. In a natural state, the creek would tend to migrate back and forth across the floodplain. Since many houses are located right next to the creek, it has been forced to remain in a fixed location.  The roads in the canyon also confine the creek to a fixed position, and in so doing, they are put under significant risk of being eroded by the creek during floods.  The houses and roads are stabilized and reinforced by rip rap, gabions (wire structures filled with rocks), or concrete structures. However, these structures are still susceptible to erosion and can be undermined by repeated flooding.

Grazing and logging practices along Upper Mill Creek significantly increase the risks associated with flooding and mass wasting.  Both practices destabilize the slopes on which they occur.  Large logging equipment is needed in order to harvest trees from a slope.  In order to access the slope with such equipment, roads must be built.  Logging roads constructed on steep slopes increase the chance of mass wasting events occurring by over-steepening the slope.  In addition to the over-steepening associated with road building, the practice of logging increases the erosion of the slope.  Vegetation on the slope is destroyed by the dragging of logs along the ground.

Cattle grazing on steep slopes may have a similar effect as logging; overgrazing destroys the vegetation that stabilizes the slope.  These two land uses are the major ways in which humans greatly increase the hazards related to flooding, and to mass wasting, a phenomenon which results in sediment being deposited on roads, private property, homes, and in drainage structures.  While it is true that debris flows and mudflows occur naturally in steep canyons, grazing and logging have increased the size and frequency of these events.

One such debris flow occurred in a tributary of Mill Creek in 1996, burying the lower story of an A-frame house and a section of Mill Creek Road.  The debris flow roared through the home in the middle of the night.  This debris flow originated in logging road fill, and was triggered by heavy rain on snow.  Many similar debris flows and mudflows occurred in other tributaries of Mill Creek in 1996.

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Flood History and Damages

Major floods occurred on Mill Creek in March 1931, December 1964, and February 1996.  All of these floods happened during a “Chinook” weather condition, characterized by warm temperatures and winds.  These Chinook conditions can rapidly melt snow causing flooding.  The Chinook conditions during these flood events were compounded by heavy precipitation.  Additionally, if the rain falls and the snow melts on frozen ground, then the runoff cannot soak into the ground as it normally would (CFHMP 6-2).

The 1931 flood was particularly devastating because Mill Creek flowed through the streets of downtown Walla Walla.  This flood spurred the construction of the Mill Creek Project through Walla Walla, for the purpose of flood control to protect the city from another devastating flood.

The Mill Creek Project, constructed by the United States Army Corps of Engineers, was completed in 1942.  It consists of a diversion dam, which channels water into an offstem reservoir, and a flood channel that has a high flow capacity to carry water safely through Walla Walla and College Place in the event of a flood.  During the 1996 flood, flows that exceeded 4000 cfs were diverted into the Bennington Lake reservoir.  At the peak of the flood in 1996, the water was in contact with the bottoms of the bridges throughout downtown, and the reservoir was at maximum capacity.  If the water had continued to rise, there could have been widespread damage.  The Mill Creek project has been quite successful in preventing flood damage; however, it has two main downfalls.  First, the project destroyed the creek’s habitat for fish and other critters, and second, there is no guarantee that a flood won’t overflow the channel’s banks.


Channelized Mill Creek, before and during the 1996 flood; the second picture shows the bridge from which the first picture was taken.

During the 1996 flood about 8.2 million dollars in direct damage costs occurred in Walla Walla County (CFHMP pg. 2-5).  Damage to homes and private buildings in the Kooskooskie area were widespread, as well as damage to roads, bridges, diversion structures, and levees.  Residents were stranded by the floods due to road washouts.


A home in the Kooskooskie area destroyed by the 1996 flood

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Conclusion

Though many structural engineering projects have prevented damages and reduced risk during flooding events, there will always be unforeseen risks and problems which nature can capitalize on, as long as people live, work, and play on flood plains and other environmental hazard zones (such as near steep slopes).  For the people who have chosen to live in the cool, peaceful Mill Creek canyon surrounded by nature’s beauty, there will always be a battle for survival in the face of nature’s fury.  The more we seek to control nature and prevent its natural course, the more we will lose in the way of property and a connection to the land.

Walla Walla County’s Comprehensive Flood Hazard Management Plan, which is still in the works, outlines the problems related to flooding in the county. It proposes ways in which problems should be addressed by the community organizations that are responsible for flood management.  Overall, the hazards associated with flooding must be dealt with in a holistic approach, so as to not overestimate the security of living on a floodplain or near a steep slope.  Most certainly, nobody living along Mill Creek will ever be completely safe from the dangers of floods.
 

“The government tells us we need flood control and comes to straighten the creek in our pasture.  The engineer on the job tells us the creek is now able to carry off more flood water, but in the process we have lost our old willows where the owl hooted on a winter night and under which the cows switched flies in the noon shade.  We lost the little marshy spot where our fringed gentians bloomed.”       -Aldo Leopold
 

Resources

Walla Walla Regional Planning Department, 1999, Comprehensive flood management plan: Walla Walla County, Washington.

1997 United States Corps of Engineers Reconnaissance Report.

U.S. Army Corps of Engineers, 1972, Flood Plain Information, Walla Walla County, Washington, Kooskooskie and Vicinity, Mill Creek: Walla Walla District.
 

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