What could be a threat to the miles of Idaho streams past the Lower Four Snake River dams? Sediment generation threatens salmon and steelhead habitat upstream. Fine sediment (particles less than about six millimeters in diameter) deposit in streams to cover the larger particles, like gravel and rocks. When the fine sediment starts to bury the larger gravel and rocks, it embeds the larger rocks in dirt. Embeddedness is the degree to which the larger rocks are buried. A zero percent cobble embeddedness means that the small spaces between the gravel and rocks are clear and the rocks’ surfaces that don’t touch other larger particles are touching water, not sediment. One hundred percent cobble embeddedness, on the other hand, is when gravel and larger rocks are completely buried with fine sediment.
Why is cobble embeddeness bad for salmon and steelhead? Salmonids release their eggs in gravel and rocks, and when they first hatch, fry use the spaces between these rocks to hide from predators while they grow before they begin their migration to the ocean. Fine sediment fill these gaps, creating a tomb for eggs and eliminating places for fry to hide from predators. The more the fine sediments are present in spawning and rearing areas, the less fish survive and the less fish are found.
What activities create fine sediment? Various forestry practices generate sediment, including operations associated with logging. However, roads generate the most sediment into streams. This includes roads built to access logging units, and this includes temporary roads. Roads deposit sediment into streams when they are constructed alongside streams, and roads can pose a landslide risk into the stream. In 1997, the Forest Service reviewed over 900 landslides over the winter and into the spring of 1995 and 1996 on the Clearwater National Forest, and found that over half of these landslides were associated with roads. And as the planet warms and steep roaded areas absorb more precipitation in the form of rain, some areas will become more prone to landslides.
Reducing the legacy logging roads is key to improving upstream habitat. Friends of the Clearwater found a planning document in the forest-plan revision file of a 2012 planning meeting that mentioned, “when steelhead were listed, the Forest generated a red/green map that identified excess roads and recommended reducing the road network by 30-40%. To date , no decision has been made to include this issue during the revision process.” We submitted a Freedom of Information Act for this map, and the Forest Service provided the following map of the Nez Perce National Forest. The red are the potential roads, generating sediment, that could be obliterated to improve salmon and steelhead habitat. Here is the full map:
You can download the above map here NPNF_PermTempRds_20000106.
To date, the Forest Service has not considered this in its forest plan revision, and many logging projects involve constructing new roads, constructing temporary roads that impact the landscape for years, or maintaining the vast road mileage already on National Forest System Lands. Logging and the roads used to access this logging constitute an ever-present threat to salmon and steelhead spawning habitat.
Barik et al. 2017. Improved landslide susceptibility prediction for sustainable forest management in an altered climate. Engineering Geology 230:104-117.
McClelland et al. 1997. Assessment of the 1995 & 1996 Floods and Landslides on the Clearwater National Forest. U.S. Forest Service Report to the Regional Forester, Northern Region.
Montgomery et al. 1996. Stream-bed scour, egg burial depths, and the influence of salmonid spawning on bed surface mobility and embryo survival. Can. J. Fish. Aquat. Sci. 53: 1061-1070.
Stowell and Espinosa 1983. Guide for Predicting Salmonid Response to sediment yields in Idaho Batholith Watersheds. U.S. Forest Service Northern Region Intermountain Region publication.
Waters, T.F. 1995. Sediment in Streams: Sources, Biological Effects and Control (American Fisheries Society, Bethesda, MD).
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