Fine sediment and salmonid production: a paradox

The term "sediment," as commonly used by fisherybiologists, means fine sediment and excludes up to 90% of sedimentarymaterial in streams. In mountainous terrain, hillslope erosion (primarilymass soil movements) provides periodic inputs of sediment into streamsystems, often during periods of high flow when two major sedimenttransport mechanisms are active: (1) suspended sediment transport and(2) bedload transport. Suspended sediment consists primarily of silt andclay-size particles that may be rapidly transported downstream andlocally deposited on floodplains and overbank storage locations or thatmay infiltrate into gravel interstices of the bed. Bedload transport,consisting primarily of coarse sands or larger particles, is complex andsporadic, and has major implications regarding channel morphology andthe quality of spawning gravels. It is greatly affected by large rough-ness elements (logs, boulders, bedrock outcrops, etc.). Hence theimpacts of sediment on fish habitat are influenced by both sedimentavailability and the subsequent routing of these materials through thechannel system.
The effects of fine sediment on aquatic life have been studiedintensively for more than three decades, both in situ and in thelaboratory. Laboratory studies have demonstrated potential negativeeffects of fine sediment on macroinvertebrates, on survival and emer-gence of salmonid embryos and alevins, and on growth of salmonid fry.But there are significant difficulties in extrapolating these findingsto the field. Nearly all laboratory survival studies have used simplifiedunnatural gravel mixtures to test incubation and emergence of salmonidfry. Also, mitigating factors in streams, such as structural roughnesselements and spawning behavior of female salmonids, complicate directfield application of laboratory studies. Nevertheless, forest practicerules designed to minimize fine sediment and turbidity in streams haveresulted primarily from laboratory studies. The relatively few studiesdealing with the effects of sediment from forest management in naturalenvironments have been less conclusive. Some negative effects observedin the laboratory also occur from acute or chronic sedimentation in thefield. The problem with interpreting the results of field studies is thatincreased fine sediment from forest management is almost alwaysaccompanied by other environmental effects. Also, field studies haveshown both increases and decreases in salmonid populations associated with forest management. The studies have generally failed to isolate theeffects of fine sediment from other habitat changes.
A more holistic view of the role of sediment in stream ecosystemsis needed. Undisturbed streams in forests have stored abundant sedi-ments in their channels and maintained an equilibrium between sedimentinput and sediment routing. An abundance of large organic debris andother roughness elements played an important role in the storage androuting of sediments. Forest management has broadly changed sedimentstorage and equilibrium in streams throughout much of the westernUnited States. The general result has been a concurrent loss ofroughness elements and accelerated routing of sediment through fluvialsystems. There is evidence that stable channels containing storedsediment and large organic debris are more productive at every trophiclevel than either degraded channels mainly devoid of sediment orchannels that are aggraded and unstable. Thus there seems to be abroad middle ground between too much and too little sediment insalmonid habitats.
Forest practice rules designed to minimize introduction of finesediment into streams are justified, but in themselves do not ensureprotection of salmonid habitats. These rules might result in improvedwater quality and a reduction in fine sediment in gravels, but they donot ensure protection of the physical structure of salmonid habitats. Infact, large losses of productive habitat have occurred while these ruleswere in force. The long-term emphasis of forest practice rules on con-trol of water quality and fine sediment must be expanded to a moreholistic view of salmonid habitat. Protection of streamside vegetationand physical structure of rearing habitat for juvenile salmonids mustbe given equal emphasis.