Establishment of sediment TMDL for the Sonoma Creek and its tributaries is required due to impairment of beneficial uses caused by land-use practices which increase the rate of sediment production to streams. Adverse impacts associated with excess sediment production include degradation of salmonid habitat, loss of channel capacity, channel destabilization, and premature loss of reservoir storage. Beneficial uses potentially affected by the increased sediment production include: cold freshwater habitat, fish migration, preservation of rare and endangered species, fish spawning, recreation, warm freshwater habitat, and wildlife habitat.
Sediment listings are primarily based on the best professional judgment of physical scientists and biologists who have worked in the Sonoma Creek watershed. A watershed habitat inventory study was recently conducted by the California Department of Fish and Game in a portion of Sonoma Creek and several of its tributaries with funding assistance from the Northwest Emergency Assistance Program. This study included identification of sections of the creek that are adversely impacted by excessive sediment deposition. However, most of the watershed has not been evaluated. Additionally, linkage to causes has not been investigated on a watershed scale. Based upon the results of studies in physically similar basins in northern and central California24, habitat impairment likely includes: a) increase in the amount of fine sediment deposited at spawning sites and in pools; b) increase in the frequency and depth of stream-bed scour; c) coarse sediment deposition which precludes re-establishment of riparian vegetation and causes changes in channel width-to-depth ratio and streambed substrate which are unfavorable to fish production. When the imbalance between sediment production and channel transport capacity is substantial, larger scale channel changes occur which may result in excessive rates of bank erosion, inadequate flood conveyance, further loss of riparian vegetation, and a reduction in baseflow persistence.
Habitat conditions in stream channels are shaped by more than sediment load however. They are shaped by the interactions of streamflow, sediment, large woody debris, and stream-side vegetation. This implies that a broader, more holistic, analytical framework is needed when the principle objective of a TMDL is salmonid recovery. Such a framework is usually is referred to as watershed analysis, as has been implemented in Washington state and Federal forest lands (Washington Forest Practices Board, 1993; Federal Ecosystem Management and Assessment Team (FEMAT), 1993).
Many local, state and federal agencies are currently engaged in salmonid recovery efforts (Sonoma County Planning, USEPA, NMFS, CDFG, CDF). Therefore it would seem logical to coordinate and/or consolidate these efforts into a unified watershed assessment to: a) avoid redundant and inconsistent regulatory directives; b) accomplish a holistic and scientifically defensible analysis, as described above; and c) use scarce resources in an efficient manner. Coordination of Clean Water Act and Endangered Species Act mandates may be politically advantageous as well because: a) land-owners get one set of regulatory directives; and b) local agencies get a blue print for endangered species and water quality management. A stakeholder involvement forum will be needed to coordinate multi-agency efforts and to encourage pro-active involvement by land owners.