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Sedimentation Ponds

There have been major erosion problems on the Bay Hill Ranch for many years. A combination of overgrazing and past tillage from farming have caused tens of thousands of cubic yards of sediment to be move from the ranch into Bodega Bay near Doran Beach. Also, this area had beavers in the past. Beavers would have built and maintained dams on these creeks that would have captured sediment and created productive fish and wetland habitat.

In 1989 the California Costal Conservancy funded a project to construct two sedimentation ponds on the ranch to keep sediment from the creek beds and hills of the ranch on the ranch and out of the steelhead supporting creek and out of Bodega Bay.

The upper pond is full of sediment and has turned into a gravelly wetland environment. We have begun excluding cattle from the flood plain to help the soils convert from gravel to a higher organic matter and more productive wetland environment. This will help the plants capture sediment in the upper flood plan instead of adding more fill to the small pond area.

The lower pond still has good water and significant sediment capacity but captured sediment has established a healthy wetland area in it’s upper reaches which we are now excluding cattle from.

The images below are from the original design for the sedimentation ponds.

Many of these sediment ponds with concrete lined spillways were built in this area. Two of our neighbors at Freestone Ranch have ponds like this that have filled in. There are several problems with this design.

  • The concrete has a design life of 50 years and there is no economically reasonable way to remove or replace them. Work in a creek is very expensive because of environmental regulation. It would probably cost hundreds of thousands of dollars to replace them at the end of their lifespan with a more modern design.
  • The concrete design is fragile and high flows tend to overflow and undermine the concrete.
  • If the dame fails the sediment they have captured can be released quickly in a large and destructive flood event.
  • The dam and pond create a fish barrier and prevent the steelhead below the dam from moving into the upper reaches of the creeks.
  • They can reduce summer streamflows. Spring water that flows into the pond in the summer may evaporate before it has a chance to reach fish bearing pools in the lower creek.
  • Creeks need some sediment and gravel flow to stay healthy and maintain their vegetation. Ponds like these can capture too much sediment and cause the creeks below them to downcut more.

The ponds do keep the soil on the ranch and they do create nice wetland habitat. The modern designs for this kind of work create step pools using large boulders instead of in stream ponds like these. Despite their challenges it is great to have water and to watch the egrets, ducks and blackbirds enjoying them.

The spillway on the lower pond cannot handle large storm flows. The water sheets over the edge of the concrete and the water undermines the concrete spillway as shown in the photos below. We have added redwood boards to direct the water back on the concrete and increase the flow capacity of the spillway. Hopefully this will keep the spillway functioning for many more years. We also hope that our improved grazing practices and work to increase vegetation in the creeks will increase water infiltration and slow runoff rates in large rains and also help protect the spillway.

The stream below the upper pond has continued to down cut so now the spillway has a five foot drop at the bottom end which creates large water forces that increase erosion and undermine the bottom of the spillway. We have added rock and logs to help prevent it from failing over time

Cheney Gulch

This is where the erosion from the ranch delivered it’s sediment. This is the outlet of Cheney Gulch into Bodega Bay. There a small park, bird habitat and the Bodega Bay sewage treatment plant here.

Bodega Bay

Bodega Bay needs to be dredged regularly so that boats can continue to navigate the harbor. Sediment runoff from the land draining into Bodega Bay. Presumably agriculture in the hills that drain into the bay has been a significant contributor to sedimentation in the bay.