Evidence for historical anadromy in dam-obstructed watersheds of the Pacific Northwest

Background/Question/Methods: Hydropower development in the Pacific Northwest has greatly reduced available habitat for spawning salmon. For example, the Columbia River watershed is home to ~75 prominent dams and at least an order of magnitude more minor dams. For dams built on smaller tributaries in the late 1800s/early 1900s, records regarding use of upstream habitat by spawning salmon are often lacking. Since natural falls are strategic sites for hydropower development, in many cases there is controversy about whether natural runs of anadromous salmonids ever existed above now-dammed falls. Resolving this controversy for individual systems becomes critical in decisions by the Federal Energy Regulatory Commission about whether to relicense aging dams. Our research investigates the possibility of pre-dam anadromy in watersheds of northcentral Washington. We use paleolimnological methods to investigate whether there is evidence that freshwater systems upstream of natural falls  supported natural runs of anadromous salmonids prior to dam construction around the turn of the 19^th/20^th century. Site selection targets small lakes likely to have provided adequate spawning and rearing habitat for one or more species of anadromous salmonids. Results/Conclusions: In the Lake Chelan watershed, two surface sediment cores from a small productive upstream lake display pre-dam stable nitrogen signals consistent with the presence of anadromous salmonids in the basin (mean δ¹⁵N = 5.21±1.58 ^o/oo and 6.30± 1.58 ^o/oo).  Post-dam sediments have lower values generally consistent with salmon-free conditions (2.55±0.6 ^o/oo and 3.73±1.11^o/oo).  The record is noticeably different in a similar nearby lake unlikely to have supported natural runs of salmon. Analysis of raw sediment smears as well as diatom community composition indicate that around the time of damming, Wapato Lake changed from a turbid, productive lake dominated by blue green algae and Stephanodiscus parvus toward one dominated by green algae with significant representation of Fragilaria crotonensis. Stable isotope analysis on one sediment core from a small lake upstream of the Enloe Dam on the Similkameen tributary to the Okanogan River suggest a similar situation. Interestingly, in both cases the immediate pre-dam period was characterized by a slow decline in δ¹⁵N that then declined further upon damming, with no evidence of recovery to higher levels during the post-dam period. While salmon apparently used these systems historically, the currently unavailable habitat may have been of less importance immediately prior to damming than it may be now, when a large proportion of historic spawning/rearing habitat is inaccessible.