Seminar Abstract

Tim Beechie

Wednesday, May 13, 2005

Some potential impacts of climate change and altered runoff regimes on riverine ecosystems

Tim Beechie (1,2) and Bob Naiman (1,3)
(1) Coastal Rivers Research Consortium, (2) Northwest Fisheries Science Center, NOAA Fisheries, (3) University of Washington

Recent climate change has decreased snowpack and shifted stream flow timing throughout the western US in the past century. Discharge patterns of many mid-elevation streams were dominated by spring snowmelt through the early 20 th century, but are now dominated by rainfall runoff floods. Consequently, runoff in many streams occurs one to two months earlier, and projections indicate even more severe changes expected over the next 40-50 years. Spring flows are critical for anadromous fishes - especially steelhead - that time migration and spawning to coincide with optimal flows and temperatures, for riparian vegetation that relies on substantial spring freshets for forest renewal, and for recruitment of large woody debris (LWD) that forms high quality fish habitat in channels. We hypothesize that shifts in runoff timing have inhibited successful recruitment of cottonwood because flows now occur much earlier in the spring. Water tables decline either too early (before seeds have germinated) or too quickly (leaving roots dry during seedling establishment). Similarly, small tributaries that formerly has sufficient water for steelhead spawning now have too little water for successful spawning or egg incubation, leading to contraction of the steelhead spawning range.

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