• Historical accounts and stream surveys conducted in the 1960s by CDFG indicate that the Mattole Basin historically supported relatively robust populations of Chinook salmon, coho salmon, and steelhead trout. Fishery surveys have been conducted on many tributaries throughout the Mattole Basin in the last ten years. These biological stream surveys indicate the presence of Chinook salmon and steelhead trout in all five Mattole subbasins and the presence of coho salmon in the Eastern, Southern, and Western Subbasins. Coho salmon also utilize the Estuary Subbasin on their migrations; however, in limited surveys conducted in the Northern Subbasin since the 1980s, coho salmon have not been detected. No studies have been conducted to estimate subbasin or tributary specific population levels of coho salmon or Chinook salmon. However, a nine-year intensive study of three tributaries within the Northern Subbasin indicated stable age classes of steelhead trout. Intensive studies of the Estuary Subbasin have shown depressed populations of over-summering Chinook salmon and steelhead trout, and no coho have been detected. Mattole Basin-wide population estimates indicate depressed meta-populations of Chinook and coho salmon. A metapopulation is a “regional (Mattole Basin) population consisting of semi-isolated local (stream/subbasin) populations” (Levins 1970).

What are the current salmonid habitat conditions in the Mattole Basin? How do these conditions compare to desired conditions?

Erosion/Sediment

• Instream sedimentation in several stream reaches throughout the basin may be approaching or exceeding levels considered unsuitable for salmonid populations. Currently, the estuary is very shallow and lacks channel complexity. Erosion/sediment reduction is the top recommendation category for the Eastern and Estuary subbasins;

• High summer water temperatures in many surveyed tributaries are deleterious to summer rearing salmonid populations in the Estuary, Northern, Eastern, and Western Subbasins. Riparian/water temperature improvements is the top recommendation category in the Northern Subbasin;

• In general, pool habitat, escape and ambush cover, and water depth are unsuitable for salmonids in many mainstem and tributary stream reaches in the Mattole Basin. In the Southern Subbasin summer flow is inadequate or non-existent in many reaches. Large woody debris recruitment potential is poor in the Northern, Eastern, and Western subbasins. Instream habitat improvement is the top recommendation category in the Southern and Western subbasins;

• Available data from sampled streams suggest that suitable, high quality spawning gravel for salmonids is limited in some streams in all subbasins;

• Table 1 summarizes subbasin salmonid refugia conditions:

What are the relationships of geologic, vegetative, and fluvial processes to natural events and land use history?

• Geologic units within the basin can be grouped into one of three bedrock terrains (hard, moderate, and soft) and one for Quaternary alluvial units. Larger landslides are more prevalent in soft terrain and are typically earthflows, while smaller slides, typically debris slides, are more prevalent in hard and moderate terrains;
• Weak geologic materials, steep slopes, high rainfall, and strong earthquakes common to the basin result in high rates of natural landsliding and surface erosion, particularly in soft terrain. These natural processes can be exacerbated by human land use within the basin. About one half of the basin is considered to have a high to very high landslide potential;
• In general, the subbasins can be ranked in terms of relative impacts with geologically unstable areas linked to adverse stream effects. The Northern Subbasin has the largest proportion of geologically unstable (soft) terrain, which is linked to the highest amount of historically active landslides, gullies, and stream features indicative of excess sediment production, transport, and storage. The Southern Subbasin has the lowest proportion of geologically unstable terrain, historically active landslides, gullies, and stream features indicative of excess sediment production and transport. The Eastern and Western Subbasins are intermediate between these two extremes due to the variability in the proportion of soft terrain and steep slopes;
• Source and transport reaches of the blue line streams as depicted on NCWAP stream network maps, were identified primarily in bedrock terrains, while response (depositional) reaches were identified in the Quaternary (alluvial) unit reaches. Features indicative of excess sediment production, transport, and storage have decreased throughout most of the basin in the period between 1984 and 2000. The reduction in these features was greatest in the hard terrain. The distribution of these features in bedrock terrains suggests that portions of the areas interpreted as having a high to very high landslide potential are also the sources of sediment that has been delivered to streams;
• Human activities such as timberland conversion to grasslands and brush, grazing, timber harvest, and road construction and use, have interacted with natural geologic instability to increase sediment production above naturally high background levels. Historic timber harvesting and streamside road construction reduced riparian canopy and increased direct sediment inputs and water temperature. Overall, the current landscape is comprised of smaller diameter forest stands than in pre-European times. Decades of fire suppression have created dense forest stands and brush-lands leading to the designation of Mattole Basin population centers as high wildfire threat areas.

How has land use affected these natural processes?

• Land use, including road construction and use, timber harvesting, and grazing, have added excess sediment to the fluvial system. Many of the effects from these activities are spatially and temporally removed from their upland sources. Excess sediment remains in the Mattole mainstem despite decades of low timber harvesting activity;
• Currently, roads are a major land use contributor of sediment (CDF, 2002). Large storms or other catastrophic events combined with poor road location and construction practices have the potential to deliver large and adverse amounts of sediment into stream systems;
• Water extraction for agriculture, road maintenance, and residential use has the direct effect of reducing the amount of available habitat for fish;
• Large woody debris recruitment potential is limited by the low percentage of near-stream forest stands containing trees in large diameter classes;
• Grazing is widespread on privately owned grasslands and has shifted to cattle from sheep since the enactment of predation protection measures. Stock impacts to streams are not widespread, but watercourse exclusionary fencing is limited.

Based upon these conditions trends, and relationships, are there elements that could be considered to be limiting factors for salmon and steelhead production?

Based on available information for the Mattole Basin, the NCWAP team believes that salmonid populations are currently being limited by:

• Impacted estuarine conditions;
• General basin-wide lack of habitat complexity;
• High instream sediment levels;
• High summer water temperatures;
• Reduced basin-wide coho and Chinook meta-populations.