ABSTRACT: Effects of climate change on Pacific Northwest rivers.

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The effects of winter climate in the mountains dominate the variability of stream flow in Pacific Northwest (PNW) rivers. For many PNW rivers the seasonal variability of flow is also strongly influenced by snow accumulation and melt. Rivers at lower elevation, such as coastal streams west of the Cascade Mountains, respond directly to precipitation in winter, and runoff essentially follows the timing of precipitation. Moderate-elevation rivers in the transient snow zone behave like rain-dominated streams in fall and early winter, showing a fall peak in streamflow. Then flows dip as precipitation turns to snow in mid-winter, followed by another peak in runoff in spring from snowmelt. These basins have a "dual-peaked" hydrograph. The Columbia River, by comparison, is a snowmelt-dominated river showing a single peak during the spring freshet. Temperature plays a key role in the timing of flow in snowmelt-dominant and transient-snow river basins. Temperature changes as small 1°C, for example, can have significant impacts on water availability in the summer months even in the absence of changes in winter precipitation or annual flows. Many uncertainties regarding the exact nature of climate change remain unresolved, but all climate models project increased temperatures over the next 100 years due to increases in greenhouse gases, and this result is projected with relatively high certainty. Even the least sensitive of the Global Climate Models (GCMs) show warming on the order of 3°C by the end of the 21st century for middle of the road emissions scenarios. Regional temperatures in the PNW are also projected to rise, and annual average temperatures are expected to be on the order of 1.7°C warmer by about 2025 and nearly 2.5°C warmer by about 2045, according to recent GCM scenarios. Even in the absence of changes in winter precipitation, these temperature changes have significant implications for PNW water supplies. Simulations of streamflow in the Columbia River, for example, show that winter flows would increase, summer flows would decline, and significant summer droughts would occur two to three times as often in the future as they have in the historic record. In moderate-elevation basins with winter temperatures close to freezing (such as the Cedar River, which supplies two-thirds of Seattle’s water) reductions in average summer streamflow of about 25% could occur by the 2020s and by the 2040s severe drought years like 1992 could become the norm in these watersheds. Because winter precipitation is the primary driver for annual flows, with about 75% of precipitation in most areas falling in winter, relatively uncertain changes to winter precipitation could exacerbate or mitigate the effects of temperature-driven changes in streamflow timing. Changes in summer precipitation may also have impacts, but usually result in relatively minor changes in streamflow.Many GCMs suggest small increases in winter precipitation in the PNW on the order of 10%, but a few models project more dramatic changes. The British HadCM2 GCM, for example, predicts strong increases in winter precipitation for the PNW (particularly in the 2020s), whereas the German ECHAM4 GCM predicts strong decreases in winter precipitation. The HadCM2 simulations suggest an increased risk of flooding in November and December in moderate-elevation basins and moderate decreases in water supply in summer, whereas the ECHAM4 scenarios imply severe shortages in summer water supplies, with relatively small increases in fall and winter flood risks. The primary vulnerability of PNW water systems is limited reservoir storage and the historic reliance on natural storage in the mountain snowpack to provide sustained water supplies in the relatively dry summers. In the Columbia Basin, for example, reservoirs can store only about 30% of the annual flow, and some water supply systems in the Cascades can store only 10% of the annual flow. This limited ability to move water from the wet winter to the dry summer makes these water systems vulnerable to the potential loss of snowpack. In systems with large amounts of storage (some hydropower systems in northern British Columbia, for example), the implications of timing shifts in streamflow may be negligible, although these large storage systems may still be vulnerable to significant reductions in average precipitation or changes in the duration or intensity of droughts. The Columbia Basin is somewhat unique in the PNW because climate change will likely affect important transboundary issues between Canada and the US. The lack of a formal cooperation mechanism for ensuring instream flow in the lower Columbia Basin in summer, for example, suggests that it may be difficult or impossible for the US to avoid impacts to instream flow in the lower river in late summer if natural flows in the southern part of the basin decline as expected. Quantifying and Evaluating the Effects of Climate ChangeFor the 2020s, temperature increases of about 1.7°C are suggested for the PNW, and by the 2040s we could see temperatures increase by 2.5°C on average. The main impact for this area will be loss of snowpack. These changes are most evident in the southern Cascades, where losses of April 1 snowpack could be nearly 50% by the 2040s; however, reductions in basin average snowpack for the Columbia Basin as a whole are present in all the scenarios as well, even for those scenarios that show increases in winter precipitation. In broad terms, basins currently close to the snowline in mid-winter are at greatest risk in the short term, and we expect to see significant impacts in the US earlier than in Canada. These discrepancies in the location, timing, and intensity of impacts suggest increased tension between Canada and the US over water resources in the Columbia Basin.Water Resources in the Columbia River Basin The Columbia reservoir system is operated primarily for conjunctive winter hydropower production, and flood control in spring and summer. Reservoirs are currently evacuated in winter and refilled in the spring and summer. Other important uses of the dams and reservoirs include irrigation, instream flow enhancement for fish, river navigation, lake recreation, etc.Middle of the road climate scenarios for 2040 predict modest reductions in winter power production capability, but more significant reductions in fish flows, particularly in late summer. This suggests intensified tradeoffs between winter hydropower production and attempts to maintain objectives such as instream flow for fish, for example.For more extreme climate scenarios in which we see increases in temperature and reductions in precipitation, we see increases in drought frequency and severity. Such scenarios create severe challenges in attempting to maintain system reliability for power generation, flood prevention, irrigation, recreation, and fish flows. A recent study also shows that there is no easy way to mitigate these changes in flow by changing reservoir operations. We can isolate and protect one use at a time, but not more than one. By 2050 – 2098, we can no longer correct for additional water uses at no cost to "firm power", and we see definite tradeoffs between winter and summer objectives. There are several areas of concern for water management in the Columbia Basin: Limited reservoir storage available, and little opportunity to build more (storage / streamflow ratios 10% - 30% in most basins—vulnerable to timing shifts) Water systems operated closer to their supply limits now than in the past (effective management more important) Use of historic streamflow record for long-range planning (not representative of the future) Use of statistical streamflow forecasting tools based on 30-year streamflow record Inflexibility and fragmentation of water management institutions and entities Different changes in Canada and US that may disrupt existing management framework and agreements. As human population in the region grows, water systems will have to be operated closer to their limits, with emphasis on conservation and demand management. This is precisely why we want water managers to use streamflow scenarios that include climate change as a factor. The greatest impacts to the Columbia system are for the warm/dry scenarios, which produce the strongest reductions in summer streamflows and the greatest increases in drought frequency. In the lower Columbia Basin, summer streamflows will likely be strongly eroded by climate change. With most of the snowpack and roughly 50% of the reservoir storage in the Columbia in Canada, the need for coordination is clear. Yet currently we have no formal coordination mechanism for maintaining instream flow and reservoir elevations. How do we adapt? Whether precipitation increases or decreases, we are likely to see reductions in summer water availability due to climate change and increased regional temperatures. The reductions in water availability in these scenarios are likely to exacerbate existing conflicts over water, the impacts of regional growth, and weaknesses in infrastructure, water management practice, and management institutions. Conferences like this one in which different sectors discuss adaptation strategies become all the more important in this context

Bibliography of Canadian Geomorphology