Key Findings

Historical accounts indicate that the Estuary Subbasin supported populations of Chinook salmon, and steelhead trout throughout the summer months, in addition to being a vital transitional step on the seaward migration of juvenile salmonids and the returning spawning migration of adult salmonids. Biological studies were conducted in the estuary in the late 1980s and early 1990s by HSU researchers and the Mattole Restoration Council along with current population counts by the Mattole Salmon Group. These studies indicate that over-summering Chinook salmon and steelhead trout populations in the Estuary Subbasin are currently depressed;

Instream sediment from both past land use and natural geologic processes upstream has been delivered to the estuary by large storm events, impacting the low gradient estuarine channel. Comparison of 1942 and 1965 photos indicates that the estuary widened, and areas of vegetation were lost during that time frame. However, the 1984 and 2000 aerial photos show some channel narrowing and vegetative improvement during this time period. Whereas dormant landslides, steep terrain and areas with high to very high landslide potential indicate that slopes in the subbasin are susceptible to landsliding and erosion, the bulk of excess instream sediment appears to have been transported from upstream sources;

Soil disturbance associated with several agricultural and development activities have exacerbated the naturally high levels of sediment delivery to the Mattole River and its tributaries. In particular, vegetation removal and road construction during the post 1950 peak timber harvest period, coupled with the transport energy of the devastating floods of 1955 and 1964 have created extensive negative stream characteristics in the lower reaches of many large tributaries including mainstem Honeydew Creek. These negative impacts include displaced riparian vegetation; wide, aggraded channels; and very warm summertime water temperatures. These impacts have become resident in the Estuary Subbasin;

The present state of estuarine habitat is limiting the successful production of salmonids, especially Chinook salmon. Based on known salmonid temperature suitability studies, current sediment, and temperature impacts in the estuary are thought to be deleterious to summer rearing salmonid populations. Results of habitat assessment conducted from 1988 through 1994 in the estuary by Humboldt State University, Mattole Restoration Council, and Mattole Salmon Group researchers identified a critical shortage of adequate pool habitat, water depth, substrate embeddedness, and escape and ambush cover. These are all necessary for survival of salmonids in the critical over-summering life stage;

Although lack of escape cover for fish increases the risk of predation by birds, mammals, etc., data from other river systems indicate that seal and sea lion predation is usually not limiting to salmonids. These data indicate pinnipeds are not likely to have a large impact on Mattole Basin salmonid runs.

Recommendations

Continue to support the Mattole Salmon Group’s Chinook juvenile rescue rearing and fish-tagging efforts, and incorporate a program to monitor effectiveness;

Reduce sediment deposition to the estuary by supporting a basin-wide road and erosion assessment/control program such as the Mattole Restoration Council’s Good Roads, Clear Creeks effort;

Avoid potential sedimentation directly into the estuary from the estuary’s upland slopes, which are predominantly mélange bedrock and dormant landslides. Encourage the use of appropriate Best Management Practices to achieve this objective;

Consider the nature and extent of naturally occurring unstable geologic terrain, landslides and landslide potential (especially Categories 4 and 5, page 89) when planning potential projects in the subbasin;

Maintain and enhance existing riparian cover. Use cost share programs and conservation easements as appropriate;

Support ongoing local efforts that monitor summer water and air temperatures on a continuous 24-hour basis to detect long-range trends and short-term effects on the aquatic/riparian community;

Support efforts to determine the role of the mainstem Mattole River in elevated estuarine water temperatures;

Utilize Humboldt State University studies conducted in the early 1990s as baseline information to periodically monitor trends in estuarine conditions and fish production;

Protect instream flows in Mill Creek (RM 2.8) and Stansberry Creek for thermal refugia;

It would be informative to further study the degree to which the cool, summer base flow from Mill Creek (RM 2.8) could temper the warmer mainstem Mattole River waters and provide an area of cool water refugia. To do so, a summer low flow connection between Mill Creek and the river would have to be established through the Mattole’s gravel floodplain.