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~ 62 P. B. HOLDEN The effects of regulation discussed above have generally con- sidered negative impacts. A few situations exist where conditions for fish have been improved by regulation. The constant Summer irrigation flows and warmer winter temperatures of some Pacific Northwest (U.S.A.) streams have actually increased native salmonid production. The perturbations discussed above are the major documented factors affecting riverine fish ecology below dams. These effects are fairly well known and are also generally quite predictable if sufficient life history information is known about the riverine fishes present. DeLayed Impacts of ReguLation Information from several areas suggests that some of the effects of dams may noe appear immediately after closure, but tend to slowly appear several years later. The general effect of these impacts on riverine fishes is a rather slow reduction in populations rather than the immediate loss usually associated with dams. These so- called delayed impacts are poorly understood and have not been well documented. Therefore, the following discussion should be viewed as a series of hypotheses that need testing before they can be considered valid. Also, many of these factors operate many miles below the dam, usually outside the boundaries of studies intended to delineate downstream effects of a dam. One delayed impact is the habitat change that occurs after several years of low or reduced flows in areas far downstream from the dam itself. Reservoirs generally tend to store water during high spring flows, which means that yearly high flows are generally missing. The loss of these flows, which are needed for scouring and channel maintenance, may be especially important in maintaining fish habitat. Reduced flows in some areas may reduce the braidedness of a channel, but perhaps increase it in other situations. Channels, therefore, may become deeper or, perhaps, shallower, changing the type and quantity of fish habitat. I suspect these changes do occur, and that most are detrimental to native riverine fishes, but there are few data presently available to support this hypothesis. It is fairly well understood that the amount of fine sediments below dams is generally reduced because the reservoir acts as a silt trap. The effect of reduced silt deposits, and the consequent armoring, may affect the variety of habitat available to fishes. In general, it can probably be correctly stated that these processes tend to reduce the habitat variability below dams, especially the slow, silt substrate areas usually preferred by young fishes. The distance below a dam such phenomena may take place depends on the