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Documentation of the interacting effects of river regulation and climate on riparian vegetation has typically been limited to small segments of rivers or focused on individual plant species. We examine spatiotemporal variability in riparian vegetation for the Colorado River in Grand Canyon relative to river regulation and climate, over the five decades since completion of the upstream Glen Canyon Dam in 1963. Long-term changes along this highly modified, large segment of the river provide insights for management of similar riparian ecosystems around the world. We analyze vegetation extent based on maps and imagery from eight dates between 1965 and 2009, coupled with the instantaneous hydrograph for the entire period. Analysis confirms a net increase in vegetated area since completion of the dam. Magnitude and timing of such vegetation changes are river stage-dependent. Vegetation expansion is coincident with inundation frequency changes and is unlikely to occur for time periods when inundation frequency exceeds approximately 5%. Vegetation expansion at lower zones of the riparian area is greater during the periods with lower peak and higher base flows, while vegetation at higher zones couples with precipitation patterns and decreases during drought. Short pulses of high flow, such as the controlled floods of the Colorado River in 1996, 2004, and 2008, do not keep vegetation from expanding onto bare sand habitat. Management intended to promote resilience of riparian vegetation must contend with communities that are sensitive to the interacting effects of altered flood regimes and water availability from river and precipitation. å©2015. American Geophysical Union. All Rights Reserved.

We quantified all components of a fluvial sediment budget for a discrete flood on an aggrading gravel bed river. Bed load transport rates were measured at the upstream and downstream ends of a 4 km study area on the Provo River, Utah, during a dam-controlled flood. We also collected high-resolution measurements of channel topography before and after the controlled flood for the entire reach. Topographic uncertainty in the digital elevation models (DEM) was characterized using a spatially variable approach. The net sediment flux provided unambiguous indication of storage. Sediment input to the reach (319 m3) exceeded output (32 m3), producing a net accumulation of approximately 290 m3. The difference between the scour and fill was also positive (470 m3), but uncertainty in the topographic differencing was larger than the observed net storage. Thus, the budget would have been indeterminate if based on morphologic data alone. Although topographic differencing was not sufficiently accurate to indicate net storage, it was able to demonstrate that internal erosion was a larger sediment source than the net sediment flux. The magnitude of total erosion (1454 m3) and deposition (1926 m3) was considerably larger than net change in storage, showing that internal sources and sinks were the dominant driver of channel change. The findings provide guidance for the development of sediment budgets in settings in which one must choose between a morphological approach and the direct measurement of sediment flux.

We evaluate three metrics representing the drivers of channel change downstream from dams. A balance between changes in sediment supply and transport capacity identifies conditions of sediment deficit or surplus. A Shields number represents the competence of postdam flows and the potential for incision under conditions of sediment deficit. A ratio of postdam to predam flood discharge provides a metric for the scale and rate of channel change, especially width. The metrics are calculated for more than 4000 km of some of the major rivers in the western United States. More than 60% of these rivers are in sediment deficit, and only a few reaches are in sediment surplus. The sediment balance can be used to assess the relative effort involved in reversing undesired conditions of deficit or surplus.

Unique features of the Colorado Plateau include aridity and the associated importance of water, topographic control on local ecosystems, large urban centers on the periphery but none within the region, and a combination of resource-extraction industries and tourism. Potential constraints to EM are conflicts in the legislative and administrative framework, inadequate staffing and funding of the agencies, insufficient communication among elements of the public, and the difficulty of achieving large number of monitoring and research sites, abundance of data, grass-roots ties to the land, and agency commitment. Needs for implementation are definition of EM, implementation initiated at the local level, and an atmosphere of trust and open communication.