Agency Creek enters Simcoe Creek at river mile 9.5, and drains a 23-square-mile watershed. Middle Columbia River steelhead use the creek for spawning and rearing. Steelhead redd counts in Agency Creek have ranged from 4 to 20 since 1999, and the creek is recognized as a spawning aggregation for steelhead recovery purposes. Juvenile steelhead rear through the summer in the creek near its confluence with Simcoe Creek.
Agency Creek is perched atop the Agency/Toppenish/Mill creeks alluvial fan complex and dries up in the summer and fall. During the high-flow event of February 1996, a gravel road in the middle of this alluvial fan complex captured the stream flow and diverted it down the roadbed for approximately 800 ft before re-entering the stream bed. A gravel deposit was left in the channel at the point of diversion which caused the creek to continue to flow down the road in winter and spring. As a result, several three-feet deep headcuts were created in the road and gradually advanced upstream toward the point of avulsion. These head-cuts posed a fish passage problem during low flows.
The project purpose was to rehabilitate the road, allowing its continued use, and prevent future erosion by:
- Remove gravel deposit that causes the stream to divert down the road
- apply grade controls to rehabilitate headcuts
The project proposal called for the removal of the gravel deposit left in the abandoned Agency Creek channel at the time of capture by the gravel road; prevent recapture of stream flow by the road; and rehabilitate the headcuts. The use of large rock was specified after taking into consideration the flashy hydrograph and the lack of floodplain confinement in this reach of Agency Creek.
The project was completed and is now in the monitoring stages. We removed the gravel plug that kept the channel from accessing the original channel. Eleven rock grade control (drop) structures were strategically placed throughout the creek to control further headcutting. These structures serve to dissipate energy, increase sinuosity, and increase channel/floodplain interactions, all of which will increase the deposition of sediment and accelerate channel aggradation. All work was done in the dry to prevent any stress on steelhead.