FISHERIES HABITAT Effects of Suction Dredging on Streams: a Review and an Evaluation Strategy

Suction dredging for gold in river channels is a small-scale mining practice whereby streambed material is sucked up a pipe, passed over a sluice box to sort out the gold, and discarded as tailings over another area of bed. Natural resource managers should be concerned about suction dredging because it is common in streams in western North America that contain populations of sensitive aquatic species. It also is subject to both state and federal regulations, and has provided the basis for litigation. The scientific literature contains few peer-reviewed studies of the effects of dredging, but knowledge of dredging practices, and the biology and physics of streams suggests a variety of mechanisms linking dredging to aquatic resources. Effects of dredging commonly appear to be minor and local, but natural resource professionals should expect effects to vary widely among stream systems and reaches within systems. Fishery managers should be especially concerned when dredging coincides with the incubation of embryos in stream gravels or precedes spawning runs soon followed by high flows. We recommend that managers carefully analyze each watershed so regulations can be tailored to particular issues and effects. Such analyses are part of a strategy to (1) evaluate interactions between suction dredging and other activities and resources; (2) use this information to regulate dredging and other activities; (3) monitor implementation of regulations and onand off-site effects of dredging; and (4) adapt management strategies and regulations according to new information. Given the current level of uncertainty about the effects of dredging, where threatened or endangered aquatic species inhabit dredged areas, fisheries managers would be prudent to suspect that dredging is harmful to aquatic resources. uction dredging for gold is a small-scale mining practice whereby streambed material is excavated from a wetted portion of a river channel and discarded elsewhere. Suction dredges use high-pressure water pumps driven by gasoline-powered motors to create suction in a flexible intake pipe [commonly 75-300 cm (3 in-12 in) in diameter]. The intake pipe sucks streambed material and water and passes them over a sluice box that is usually mounted on a floating barge. Dense particles (including gold) are trapped in the sluice box. The remainder of the material is discharged into the stream and can form piles of tailings or spoils. Large boulders, stumps, and rootwads may be moved before excavating a site, and rocks too large to enter the intake pipe are piled nearby. Dredging can vary in area from a few small excavations to the entire wetted area in a reach and can exceed several meters in depth. Material is commonly dredged from pools and cast over downstream riffle crests. Suction dredging is common during the summer in many river systems in western North America. It can affect aquatic and riparian organisms (Griffith and Andrews 1981; Thomas 1985; Harvey 1986), channel stability (T. E. Lisle and B. C. Harvey, personal observation), and the use of river ecosystems for other human activities. Bret C. Harvey is a fish ecologist and Thomas E. Lisle is a geomorphologist for the U.S. Forest Service, Pacific Southwest Research Station, 1700 Bayview Drive, Arcata, CA 95521 USA; 707/825-2926; [email protected]. On-site effects of dredging Suction dredging is regulated by both state and federal agencies, based in part on the U.S. General Mining Law of 1872, Organic Administration Act of 1897, and Clean Water Act of 1972. Suction dredging is an important issue to fisheries professionals because many dredged streams contain threatened or endangered species, and the adequacy of agency management of suction dredging has been legally challenged. Surprisingly, the effects of suction dredging on river ecosystems have not been studied extensively. A literature search yielded only five journal articles that specifically address the effects of suction dredging (Griffith and Andrews 1981; Thomas 1985; Harvey 1986; Hall 1988; Somer and Hassler 1992). However, some impacts of dredging can be predicted from general knowledge of physical and biological processes in streams. Our goals in this paper are to summarize potential effects of suction dredging on stream biota and physical channel characteristics and to propose a basin-scale strategy for evaluating the effects of suction dredging. We also identify several research areas critical to improving management of suction dredging in streams. Entrainment of organisms by suction dredges State regulations generally limit dredging to summer months, but dredging can still overlap with fish spawning and incubation of embryos. In some streams salmonids do not emerge from the substrate until summer, and many Vol. 23, No. 8 8 Fisheries