Flood Control in the Walla Walla Valley

by Chris Garratt



The Walla Walla Valley is prone to flood damage.  There are many contributing factors to this phenomenon; one large cause of flood damage is that Walla Walla was built on a flood plain.  Thus, when high volumes of water flow through Mill Creek and overcome the stream’s carrying capacity, the excess water has nowhere to go but into the City of Walla Walla.

In the 1930s, the people of Walla Walla had had enough.  Led by a man named Virgil B. Bennington, a petition was started to get federal funding to build flood control devices in and along Mill Creek.  On June 28, 1938, the Flood Control Act was passed in Congress, which called for two projects to be built in the Walla Walla Valley: the Mill Creek Flood Control Plan and the Mill Creek Channel.

Mill Creek Flood Control Plan

The Army Corps of Engineers completed the Mill Creek Flood Control Plan in 1943, the first of  two projects.  The project was created to divert as much as 4000 cubic feet per second (cfs) of water from Mill Creek in times of flood.  The project included a diversion structure that was built across Mill Creek at Rooks Park, diverting water to the south into an intake channel that ran to a reservoir (Figure 1).  A return canal was able to drain water held in the reservoir back into Mill Creek when the potential for flooding subsided.  Also included in the project was an additional diversion device located approximately half a mile downstream from the first diversion.  This device diverted water from Mill Creek into Garrison and Yellowhawk Creeks, directing the water south of the City of Walla Walla.

Figure 1: Map of the Mill Creek Diversion Plan (Army Corps of Engineers); click on image for larger picture

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Through the Water Resource Act of 1992, the Mill Creek Reservoir was renamed “Virgil B. Bennington Lake” in honor of the man who is thought to be the father of the flood control plans, Virgil B. Bennington.

Mill Creek Channel

The Army Corps of Engineers completed the Mill Creek Channel in 1948.  This project was initiated in order to channelize Mill Creek though the city, negating the effects of the alluvial fan.  Concrete barriers, walls, floors and waterfalls, as well as gabions, were constructed along its shores to create the channel.

Both structures have been proven to be successful at diverting floods from destroying the City of Walla Walla.  In 1996, for example, as much as 6,000 cfs of water flowed through Mill Creek.  Both the Mill Creek Channel and the Mill Creek Flood Control Plan were able to keep this water out of the streets of Walla Walla.

The City of Walla Walla’s Water Rights

A large portion of Mill Creek water is used to satisfy the water demands of the people of Walla Walla.  In 1866, Walla Walla received the right to divert Mill Creek water in the State of Oregon.  In 1906, the first diversion pipe was constructed.  The pipe was seven miles long and 30 inches in diameter.  The intake was located in Washington, very close to the Oregon/Washington border.  In 1922, the last four miles of the pipe line were constructed, extending the intake to its present location - four miles south of the Oregon border.  The Walla Walla watershed was established at this location.  The area is heavily protected against both human and animal trespassing.  In fact, every year, the state allows for a certain number of elk near the watershed to be killed to eliminate the population from becoming too large and destroying the watershed area.  In the 1980s the entire pipeline was replaced because the pipes where leaking, and structurally were unsound.  The current 30-inch diameter pipeline allows water to be diverted at the current intake location.  The pipe is 14 miles long and extends all the way to the water treatment plant, located just east of the Isaacs/ Mill Creek Road junction (Figure 2).

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Figure 2: Mill Creek Pipeline Diagram (Walla Walla City Waterworks); click on image for larger picture
The City of Walla Walla’s water rights are somewhat complicated.  The city is allowed to divert 28 cfs year round from the Mill Creek intake in the State of Oregon.  In less then a year, though not yet ratified, the city is to receive an additional 20 cfs from the State of Washington, but only during winter months.  During the summer months, the demand for water from the city is approximately 30 cfs, exceeding the allowable diversion by two cfs.  To meet demand, seven wells located throughout the city provide water from the deep basalt aquifer (figure 3).

Figure 3: Map of well locations (Comprehensive Water System Plan); click on image for larger picture

However, if the flow of Mill Creek goes below 12 cfs, the City of Walla Walla is not allowed to divert water. Since Mill Creek is often dry during the summer months, all of the demanded 30 cfs of water must come from the wells.  This practice created a dramatic reduction in the basalt acquifer by as much as 100 ft of water in some areas.  Some areas of the aquifer have an annual rate of decrease of three ft per year.  This reduction in water is a serious concern to the city. A new program, the Artificial Storage and Recovery Program or ASR for short, is being implemented to replenish the declining acquifer.

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Artificial Storage and Recovery Program (ASR)

An ASR facility is relatively simple.  In essence, an ASR facility is no more than a standard well installation with a provision for returning flow back down the well. Excess water during the winter months will replenish the aquifer.   In order to be effective, the quality and flow of water into the well must be carefully monitored.  The ASR system, under current plans, is to be located at the water treatment center (east of the Issacs/Mill Creek Road junction).

Water quality is extremely important. The water introduced back into the aquifer must be compatible water that meets or exceeds the quality of the existing groundwater supply.  It must also meet certain minimum turbidity levels (a measure of the level of particles within the water).  If the water that is being introduced back into the water is too high in turbidity, it could clog up the system.

Though no date has been set for the implementation of this system, the city is currently testing the system to see how effectiveness of the system.  The City of Walla Walla hopes it to be in full operational within 5 years.

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