Biological Opinions - Bureau of Reclamation

11.2.2 Factors Affecting SONCC Coho Salmon Critical Habitat
11.2.2.1 Water Diversions and Habitat Blockages
Stream-flow diversions are common throughout the species’ ranges. Unscreened diversions for
agricultural, domestic and industrial uses are a significant factor for salmonid declines in many
basins. Reduced stream-flows due to diversions reduce the amount of habitat available to
salmonids and can degrade water quality, such as causing water temperatures to elevate more
easily. Reductions in the water quantity will reduce the carrying capacity of the affected stream
reach. Where warm return flows enter the stream, fish are likely seek reaches with cooler water,
thus increasing competitive pressures in other areas.
Hydropower, flood control, and water supply dams of different municipal and private entities,
particularly in the Klamath Basin, have permanently blocked or hindered salmonid access to
historical spawning and rearing grounds. Since 1918, the completion of Copco 1 Dam (RM
198.6) has blocked coho salmon access into upstream reaches of Klamath River and tributaries.
In addition, the construction of IGD in 1961 further blocked coho salmon access upstream of RM
190. On the Eel River, the construction of the Potter Valley Project dams in 1908 has blocked
access to a majority of the historic salmonid habitat within the mainstem Eel River watershed.
As a result of migration barriers, salmon and steelhead populations have been confined to lower
elevation mainstem reaches that historically only were used for migration and rearing.
Population abundances have declined in many streams due to decreased quantity, quality, and
spatial distribution of spawning and rearing habitat (Lindley et al. 2007). Higher temperatures at
these lower elevations during late-summer and fall are also a major stressor to adult and juvenile
salmonids.
11.2.2.2 Timber Harvest
Timber harvest and associated activities occur over a large portion of the range of the ESU.
Timber harvest has caused widespread increases in sediment delivery to channels through both
increased land sliding and surface erosion from harvest units, roads, and log decks. Significant
amounts of old-growth and late-seral second-growth riparian vegetation along spawning streams
has been removed, reducing future sources of large woody debris needed to form and maintain
stream habitat that salmonids depend on during various life stages.
The potential for delivering sediment to streams increases as hillslope gradients increase
(Murphy 1995). The soils in virgin forests generally resist surface erosion because their coarse
texture and thick layer of organic material and moss prevent overland flow (Murphy 1995).
Activities associated with timber management decrease the ability of forest soils to resist erosion
and contribute to fine sediment in the stream. Yarding activities that cause extensive soil
disturbance and compaction can increase splash erosion and channelize overland flow. Site
preparation and other actions which result in the loss of the protective humic layer can increase
the potential for surface erosion (Hicks et al. 1991). After harvesting, root strength declines,
often leading to slumps, landslides, and surface erosion (Forest Ecosystem Management
Assessment Team 1993, Thomas et al. 1993).
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