Declining summer flows of Rocky Mountain rivers: Changing seasonal hydrology and probable impacts on floodplain forests

In analyzing hydrologic consequences of climate change, we previously found declining annual discharges of rivers that drain the hydrographic apex of North America, the Rocky Mountain headwaters region for adjacent streams flowing to the Arctic, Atlantic and Pacific oceans. In this study we investigated historic changes in seasonal patterns of streamflows, by comparing mean monthly flows and analyzing cumulative hydrographs over the periods of record of about a century. We tested predictions of change due to winter and spring warming that would increase the proportion of rain versus snow, and alter snow accumulation and melt. We analyzed records from 14 free-flowing, snow-melt dominated rivers that drained relatively pristine parks and protected areas, thus avoiding the effects of river damming, flow regulation, or watershed development. The collective results indicated that: (1) winter flows (especially March) were often slightly increased, (2) spring run-off and (3) peak flows occurred earlier, and most substantially, (4) summer and early autumn flows (July-October) were considerably reduced. The greatest changes were observed for the rivers draining the east-slope of the Rocky Mountains toward the northern prairies and Hudson Bay, with late summer flow decline rates of about 0.2%/year. This would have considerable ecological impact since this is the warm and dry period when evaporative demand is maximal and reduced instream flows would reduce riparian groundwater recharge, imposing drought stress on floodplain forests. In combination with the decline in annual discharge, earlier peaks and reduced summer flows would provide chronic stress on riparian cottonwoods and willows and especially restrict seedling recruitment. We predict a loss of floodplain forests along some river reaches, the narrowing of forest bands along other reaches, and increased vulnerability of these ecosystems to other impacts including livestock grazing, encroachment of upland vegetation, and weed invasion. © 2007 Elsevier B.V. All rights reserved.