Report on Water Yeild from Forest Management Letter

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the agency need not abandon its other legal duties in order to conduct a risky and, uItimately, fn::tless experiment. Logg,ing increases runoff. Removing 25-40°/a of a watershed's vegetation wiil substantially increase sediment loads reaching i:he streams. Rhodes & Purser 1998, citing Dunne and Leopold, 1978, USFS, 1980, Swanson, et al., 1987, Everest, et al., 1987; Geppert, et al., 1984; Everest et al, 1985; MacDonald and Ritiand, 1989; Platts et al, 1989; Hicks et al., 1991b. As runoff and erosion increase, large amounts of sediments and nutrients wash into lakes and rivers. Excess sedimenl:s de:grade waterbodies and impair beneficial uses (Waters 1995). • Logging increases stream channel erosion. "Increases in peakflow will increase channel erosion via channel expansion anci chsinnei extension." Rhodes & Purser 1998, citing studies on the effects of logging in Oregcin (I)ose and Roper, 1994), Idaho (King 1989), and - snowmelt dominated azeas of Ari:zona (Heede, 1991). Channel form is determined by the flow regime, the characteristics o:f secliment moving through the channel, and bed and bank materials (Brookes 1996). Channel stability depends on factors such as channel dimension in relation to flow•velocity, the roug;hness of channel materials, sediment load, and sediment particle sizes. When one or more: of these characteristics is altered such that the stream is unable to dissipate the energy of rtreamflow, increased channel erosion and channel incision result. The increase in runoff volume: from logging can force sediments too rapidly through the channel, scouring the river bmttom and disrugting habitat for bottom-dwelling organisms. • Bv increasin$ sediment loads. los;ging...increases turbiditv. Water turbidity resulting from sediment suspended in the water column has numerous effects on aquatic ecosystems (Newcombe and MacDonald 1991). Turbidity reduces the light available for aquatic algae and other primary producers to use ir.i photosynthesis. This decreases the production of photosynthesizing plants that form the base of aquatic food webs. Turbidity also causes stream invertebrates to increase their drifting movements downstream. This avoidance response depletes local invertebr;ate popuiations and the insect food supply of foraging fish. Laboratory and field experiments demonstrate a similar avoidance of turbid water by fish. Turbidity limits fish vision, resulting in reduced feeding and growth. Suspended sediments can also sett(e on the surfaces of fish gilis, impairing respiration. As a result, fish are typically absent from stream reac;hes with high levels of suspended sediment. Lo¢ging increases sediment devositi,Dn in streams bottoms. "Logging and logging roads typically accelerate sediment delivery to streams on the order of two to ten times natural rates." Rhodes & Purser, citing Geppert, et al., 1984 and MacDonald & Ritland, 1989. EYCess deposited sediment has nnany negative effects on aquatic habitat and biota (Chapman 1988; Heede and Rinne 1990). ;5ediment deposits that fill spaces within larger cobble and gravel substrates reduce habitat :For bottom dwelling invertebrates. If gravel and cobble are completely embedded by fine sediments, invertebrate populations and the abundance of invertebrate taxa available as food for fish can be greatly reduced. Embedding of spawning gravel with sediments also inhibits fish reproduction (Bjornn and Reiser 1991). For example, sediments can settle within salm.onici redds (gravel nests) and impede the flow of oxygenated water to eggs. As a result, developing embryos and sac fry are unable to survive. Moreover, 4