Impoundment effects override natural, reach-based channel geomorphology influences on seasonal waterbird distribution in Grand Canyon along the Colorado River downstream from Glen Canyon Dam. Large winter waterbird populations were rare or non-existent prior to completion of Glen Canyon Dam in 1963, and pre-dam summer breeding was rare. Post-dam river corridor surveys of 13 geomorphological reaches from 1973 to 1994 detected 58 species of waterfowl, waders, shorebirds and piscivorous raptors, with a grand mean of 13812 waterbirds=reach (SE 3110, n 727 reach surveys), and a mean areaadjusted rate of encounter (AARE) of 37218 waterbirds km ΓΏ1 h ΓΏ1 of observation per reach (SE 6911). The post-dam assemblage has been dominated by Anseriformes (13 diving and 12 dabbling species) and includes regionally significant populations of wintering waterfowl and bald eagle, and breeding mallard. Most wading birds and shorebirds occur primarily as migrants or summer vagrants.
Total waterbird AARE was greatest in the productive clear water (CW) and variably turbid (VT) segments upstream from the Little Colorado River (LCR) (km 98), decreasing downstream on the usually turbid (UT) lower Grand Canyon segment. Mean total winter waterfowl AARE was 107618, and decreased by three orders of magnitude from the CW to the UT segments (p 010001). Mean total summer AARE was 217, and also decreased across the turbidity segments (p 01066). In contrast, AARE varied little between wide and narrow geomorphological reaches. Total AARE was only 114 and 113-fold greater in wide versus narrow reaches within the VT and UT turbidity segments, respectively (p < 010002). Winter AARE was threefold greater (p 010002), while summer AARE was equivalent between wide and narrow reaches. These tributary-related turbidity and geomorphological reach width factors contributed to a non-linear, circuitous shift in the waterbird assemblage over distance downstream from the dam, differentially affecting the seasonal distribution of waterbird feeding guilds. We discuss flow regulation and habitat management implications.