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Estimating Erosion

A key goal at Bay Hill Ranch is to reduce ongoing soil erosion and improve the function of the watershed. The two parts of this goal are to keep soil on the hills and to encourage vegetation in the riparian areas that will turn them into sponges that hold sediment and water instead of water flowing over bedrock in deep channels. This land has seen some big changes since transitioning from Native American management to tillage, sheep and cattle grazing. We can learn and get ideas by guessing at some of the changes.

How Much Soil Has Been Lost?

There are two primary sources of soil erosion on the ranch. They are channel erosion and surface erosion.

Channel Erosion

Channel erosion is downcutting of various drainages and riparian areas. We can estimate erosion in the drainages by looking at the shapes of creeks. When we see incised channels with steep banks and flood plains above, we can guess that at some point in the past, there was only a small channel and water used to flow out onto the channel onto a floodplain. Looking at the volume of the material that would have filled the channels, we can estimate topsoil loss.

Surface Erosion

There was also tillage and overgrazing that has caused a simple reduction in depth of the top soil. Some of this soil was blown away and some flowed into creeks with rain.

There is a particular 13 acre ridge top section on the ranch with very shallow and infertile top soil and flattened edges that suggest that it was heavily tilled in the past. Known crops in the area include small grains, hay and potatoes. Many potatoes were grown in the area during World War II. The bent ripper rusting in a remote field on the ranch was probably used for potato farming.

Over grazing where sheep remove the protective vegetation layer on the soil also caused surface erosion. One corner of the soil around the 150 year old barn on the ranch is down to the bedrock that supports the foundation of the barn. I suspect this area lost 4 feet of top soil from the impact of sheep around the barn over the years.

A Guess at Volume

Below are very rough estimates using a combination of Google Earth, and on ground observations to estimate where historic flood plains were. The very rough estimates suggest that if 400,000 cubic yards of sediment eroded from the Bay Hill Ranch into the 780 acre Bodega Harbor, it added an average of 4 inches of sediment to the harbor. The actual erosion could be higher than this because these estimates do not include any overall surface erosion or erosion from slides caused by upland erosion and the down cutting of drainages.

We can extrapolate the estimate from this ranch to the overall Bodega Bay watershed. There is roughly 4000 acres of watershed draining into Bodega Bay. If that watershed contributed on average 5 inches of soil erosion to the bay which is what our estimates suggest on the Bay Hill Ranch, that deposits a roughly 2 foot layer of sediment in the harbor over 150 years. That is 2.5 million cubic yards of soil moved from the hills to the bay. Dredging 110,000 cubic yards of sediment from the harbor costs $4.4 million.

Bay Hill Erosion Estimate Avg Width – ft Avg Depth – ft Cross section – sq ft Length – ft Volume-cu yd
Above Upper Pond
Upper main branch  50  25  625  600  13,889
Side Branch  60  25  750  650  18,056
Lower  55  20  550  800  16,296
Above Lower Pond  80  30  1,200  2,188  97,244
Eucalyptus Channels  90  25  1,125  2,600  108,333
Below Lower Pond  70  25  875  3,600  116,667
Area – sq ft Depth – ft
Upper tillage area  584,000  2  43,259
Around Barn  6,000  3  556
Bodega Bay Area – acres Area – sq ft Sediment Volume – cu ft Depth Added – ft
Bodega Harbor Area  781  34,000,000  11,186,100  0.33