Ann. N.Y. Acad. Sci. ISSN 0077-8923 ANNALS OF THE NEW YORK ACADEMY OF SCIENCES Issue: The Year in Ecology and Conservation Biology Dams in the Cadillac Desert: downstream effects in a geomorphic context John L. Sabo, 1 Kevin Bestgen, 2 Will Graf, 3 Tushar Sinha, 4 and Ellen E. Wohl 5 1 School of Life Sciences, Arizona State University, Tempe, Arizona. 2 Larval Fish Laboratory, Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, Colorado. 3 Department of Geography, University of South Carolina, Columbia, South Carolina. 4 Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, North Carolina. 5 Department of Geosciences, Colorado State University, Fort Collins, Colorado Address for correspondence: John L. Sabo, School of Life Sciences, Arizona State University, P. O. Box 874501, Tempe, AZ 85287. John.L.Sabo@asu.edu This paper was motivated by the 25th anniversary of the publication of Marc Reisner’s book, Cadillac Desert: The American West and its Disappearing Water . Dams are ubiquitous on rivers in the United States, and large dams and storage reservoirs are the hallmark of western U.S. riverscapes. The effects of dams on downstream river ecosystems have attracted much attention and are encapsulated in the serial discontinuity concept (SDC). In the SDC, dams create abrupt shifts in continua of downstream changes in physical and biotic properties. In this paper, we develop a framework for understanding how channel geometry and network structure influence how the physical components of habitat and the biota rebound from discontinuities set up by large dams. We apply this framework to data describing the flow regime, temperature, sediment flux, and fish community composition below Garrison Dam on the Missouri River, Glen Canyon Dam on the Colorado River, and Flaming Gorge Dam on the Green River. Sediment flux in dam tailwaters is under strong control by channel geometry. By contrast, dam-related changes in temperature and flow variation are not significantly modulated by channel geometry or tributary inputs if flow volumes are small (Missouri and Colorado River tributaries). Instead, small tributaries provide near-native conditions (flow and temperature variation) and, as such, provide key refuges for biota from novel habitats in mainstem rivers below large dams. Unregulated tributaries that are large relative to their respective mainstem (e.g., Yampa River) provide refuges as well as significant amelioration of flow and temperature effects from upstream dams. Finally, the proportion of native fish increases with distance from dam and exhibits sharp increases near tributary junctions. These results suggest that tributaries—even minor ones in terms of relative discharge—act as key refugia for native species in regulated river networks. Moreover, large, unregulated tributaries are key to restoring continuity in physical habitat and the biota in large regulated rivers. Keywords: dam; river; valley form; network; tributary; sediment; temperature; flow anomaly; fish; nonnative species Dams in the Cadillac Desert This paper was motivated by the 25th anniversary of the publication of Marc Reisner’s book, Cadil- lac Desert: The American West and its Disappearing Water . 1 This book is among the top 100 nonfic- tion titles published in the English language. One of the central theses of Cadillac Desert is that the vast quantity and storage capacity of dams and reser- voirs in the West is simultaneously a testament to the success of human engineering of landscapes and a mirage that obscures the true costs of such projects. Water projects have greened large parts of west- ern deserts and provide water for farms that gen- erate high-quality produce, tap water to some of the fastest-growing U.S. cities, and are a critical cush- ion for this new society against regular droughts. The investment costs for these services were high and typically underwritten by taxpayers but were offset by the services they provide. However, the ex- ternal costs of these projects were not fully consid- ered and are still largely ignored. These externalities doi: 10.1111/j.1749-6632.2011.06411.x Ann. N.Y. Acad. Sci. 1249 (2012) 227–246 c  2012 New York Academy of Sciences. 227