Sediment and our Streams
The decline in salmon and steelhead in the Pacific Northwest is attributable in part to sedimentation. The federal government has listed the Navarro River watershed as an "impaired water body" for both sediment and temperature. Silt and sand create embedded spawning conditions and the larger gravel, cobbles and boulders fill in deep pools and alter the shape of the stream. Destabilized stream banks and shallow, wider stream channels can increase water temperatures. Incised channels--streambed cuts into the valley bottom, and streambed aggradation--level of the streambed rises because of the input of all the fine and course sediments, are the result of excess sedimentation in the stream system.
Soil types have a huge impact on the rates of erosion and sediment production. In the Navarro watershed we have three basic soil types. In much of Anderson Creek as well as in the upper and middle Rancheria Creek and also a portion of Indian Creek, we have a highly erodable soil type called Franciscan Melange. In Anderson Valley and in the low-lying flood plain areas we also have Alluvial Fill. Alluvial fill is also highly erodable. Despite its susceptibility to flooding it is a "rich" soil and good for agriculture. The rest of the watershed is known as the Coastal Belt of the Franciscan Assemblage. These soils are more stable and less erodable than the Franciscan Melange or the Alluvial fill.
When the Navarro Restoration Plan was written the overall estimated rate of erosion from all the streams in the Watershed was just under 500.000 tons of sediment a year. Apparently this is an improvement from the period between the 1950s and 1970s, most likely due to changes in forestry practices, regulations and road construction and maintenance. However, our present day rates are still undesirably high for the overall health of the watershed and the fish.
Stream and soil dynamics are continually changing. Indian Creek, mainstem Navarro, lower Rancheria and the North Fork Navarro are all recovering from channel aggradation that began in the 1950s and 1960s. These streams seem to be recovering as their channels are narrowing and the beds are scouring from flood waters moving out the sediments further down the system or all the way out to sea. Stream bed aggradation or the actual rise in the stream bed, is still happening in a few of the major tributary systems where the gradient or slope of the stream is gentle and the valley width is large and the stream is receiving more of a sediment load than it is able to move through. You can see this process at work in the Anderson Creek valley and the upper and middle reaches of Rancheria Creek. Other Streams are actively degrading. Incised channels are where the slope, velocity, and discharge are such that more sediment is being transported out than is being delivered in to that stretch of stream. We see this process at work in Robinson Creek as it runs through downtown Boonville.
The most pervasive sediment problem affecting this stream system is the continual input of fine sediments which is widespread throughout the watershed. Roads are one of two major sources of sediment delivery into the stream system. The density of roads varies throughout the watershed and depending on the soil types and land use patterns some roads may be more problematic than others. Nonetheless, taking care of our roads is one of the primary ways we can improve stream health. The other major source of sediment is stream bank erosion and shallow landslides primarily in the alluvial valleys and the Franciscan melange terrain. Gully erosion is another source, primarily in the melange terrain and the Coastal belt grasslands and oak woodlands.
All of these affects can be witnessed as we travel around the watershed. Various restoration techniques can take care of most of the controllable sediment delivery situations previously described. Some are simpler than others. The Forest and Ranch Roads Handbook is a really good resource for road building and maintenance. The restoration field has learned a lot about constructing and maintaining roads in recent years and are continually improving on the techniques. As far as stabilizing stream banks, planting flood plain terraces with native plants grown with seed from local native species or sprigging with willow near the waters edge can make a tremendous difference. Building brush check dams in gullies with proper spacing between them can catch silt and sediment as it is being washed down.
Watching our soils being washed away can be very upsetting. Learning the early signs of potential problems is the most effective means of managing our landscape, and before we act to be aware that any changes made to one section of a stream may have impacts downstream--for better or worse. If you have questions about the best course of action it's a good idea to get input from a resource professional. Far more than anything else we do about sedimentation, by connecting our soils to our streams we can see in a relatively short time the benefits to our land and the overall health of the stream system.
Written by Linda MacElwee. Email at lsmacelwee@hotmail.com