PNW Pacific Northwest Research Station INSIDE The Geologic Divide . .2 The Allocation Challenge . .3 Dam the Consequences . .4 Land Use, Water, and Time . .4 Municipal Waters and National Forests . .5 FINDINGS issue forty-nine / december 2002 “Science affects the way we think together.” Lewis Thomas GEOLOGY AS DESTINY: COLD WATERS RUN DEEP IN WESTERN OREGON “Geology plays an extremely But a closer look reveals that the state has been blessed with a huge geological IN SUMMARY important and previously backup plan for water supply. unappreciated role in The summer of 2001 brought the “Contrary to popular belief, most summer determining flow regimes.” second-worst drought on record in flow out of the high Cascades is not due to Gordon Grant snowmelt. Instead, the high Cascades form Oregon, resulting in historically low a vast hydrologic sponge that stores many streamflows and reservoir levels, or most of the year, it just doesn’t decades worth of water as deep groundwa- seem like water is a problem in ter,” says Gordon Grant. “Recent analysis stressed aquatic ecosystems, and even F western Oregon. It’s a fixture, for of streamflow patterns, ranging in scale dramatic confrontations between irri- better or for worse. Few people, including from individual mountain streams to larger residents, fully appreciate that the warm, rivers such as the McKenzie, has revealed gators and federal resource agencies dry sunny weather of each summer actu- that geology plays an extremely important in the Klamath basin. These events ally constitutes a pretty reliable drought, and previously unappreciated role in deter- with serious implications for water mining flow regimes.” underscore the critical and growing management. By relating patterns of streamflow to importance of water availability and A rapid scan of any regional newspaper underlying geology, particularly rock age allocation in the Pacific Northwest. will uncover the critical and growing and drainage density, the extent of importance of water availability and allo- geologic control can be assessed. Knowing Yet at the scale of large river basins, cation in the Pacific Northwest. The state the geology provides a framework that can such as the Willamette River in western of the year-round supply of water relates be used to predict streamflow behavior not just to the allocation question of who under current and future climate and land Oregon, we have an imperfect picture gets what when, but to climate change, use change scenarios. In fact, not taking of how much water of what quality is dam relicensing, and whole watershed geology into account in managing regional management. water resources has been a long-standing available where and when. Having In western Oregon, the Forest Service sits mistake, according to Grant, research accurate information on the sources squarely in the midst of water manage- hydrologist with the PNW Research ment issues, simply because of the extent Station in Corvallis, Oregon. of water within a watershed, and how and location of federal lands: much of the In a region dominated by high winter and those sources vary through time in late summer flow of the region’s largest low summer rainfall, it would seem peak response to both climatic fluctuations river, the Willamette, originates in the flows and low flows of rivers would federal-dominated McKenzie drainage. simply and reliably reflect this seasonal and human activities, is vital to evalu- With steady immigration into the region pattern. Grant’s first clue to Oregon’s ating effects of forest land use, dams, likely to continue, allocation and supply of grand geological water plan was that a water are potentially long term and grow- number of rivers originating in the high diversions, agricultural and municipal ing problems facing the agency. Cascades barely even register the dry uses, and restoring instream flows. season in their hydrographs, or flow charts. BELKNAP LITTLE BELKNAP MT.WASHINGTON MT. JEFFERSON BALD DUGOUT GREEN BLACK BLACK CRATER SHIELD PETER BUTTE RIDGE BUTTE CRATER Credit: USGS Panorama of the high Cascade region from McKenzie Pass, showing a young (1500 year old) blocky lava flow in the foreground, with older ➢ vents and dissected volcanoes in the background. Note the absence of streams or evidence of stream erosion, indicating that virtually all precipitation becomes deep groundwater, emerging as springs lower in the watershed. THE GEOLOGIC DIVIDE wo distinct geologic provinces— gravity. The two separate flow regimes in the sense of one giant underground the western and the high organize the geomorphology of streams. lake,” Grant explains. “There’s a key time Cascades—form the contributing element in there too: the water in these T The drainage system under each of these slopes of the Willamette River Basin, the rivers today fell as precipitation as long as provinces also differs, despite the fact that primary drainage system of the western several decades ago. The result is that they both receive comparably high precip- slopes of Oregon’s Cascade Mountains. even during drought years, creeks and itation during the winter. In the western The western Cascades are dominated by rivers fed by groundwater will flow at Cascades the drainage delivers water into older, deeply weathered, roughly layered virtually constant discharges.” larger streams quite rapidly. In the winter, volcanic rocks, with steep, highly the large rivers of the Willamette basin If he were a planner, Grant says, he’d be dissected landscapes reflecting significant receive most of their runoff from the west- thinking a lot about this. erosion. The region is typically well ern Cascades, owing to both rain and rain drained by a dense network of streams fed on top of melting snow. A lot of rain from surface and shallow subsurface routinely indicates high rivers or flooding runoff. Purpose of to come, particularly when it falls on top The high Cascades are much younger of an extensive snowpack. PNW Science Findings geologically, higher in elevation but lower In the high Cascades, the drainage system To provide scientific information in relief, and are constructed of volumi- is less organized. This water emerges from to people who make and influence nous lava flows on top of which are springs along both the east and west slopes decisions about managing land. perched large active or quiescent volca- of the Cascade crest, fed by very large noes. Not enough time has elapsed since PNW Science Findings is published regional aquifers flowing through the monthly by: the eruptive episodes for this landscape to volcanic rock. Even after winter storms, be significantly altered by erosion. In areas Pacific Northwest Research Station the flow in these rivers does not rise appre- USDA Forest Service of the most recent volcanic activity, such ciably, in part because much of the precipi- P.O. Box 3890 as the McKenzie Pass area, blocky basalt tation falls as snow, but also because most Portland, Oregon 97208 flows dominate the view. Soil development of it is stored as groundwater. In the (503) 808-2137 is poor. The surface of these flows is summer, when flows are lowest in most highly permeable to water —there are not Sherri Richardson Dodge, Editor rivers, these large groundwater reserves a lot of lakes or streams in these areas, for [email protected] mean that the high Cascade rivers flow in example—and permeability remains high Send new subscriptions and change much the same way that they do in winter. throughout the deeper volcanic layers, of address information to until impermeable layers are reached at “In fact, it is starting to appear that high [email protected] depth, where the water collects and flows Cascades water may flow from what Carlyn Mitas, Design & layout as groundwater under the influence of amounts to a huge reservoir, although not [email protected] Science Findings is online at: United States http://www.fs.fed.us/pnw Forest Service Department of The site includes our new Science Update—scientific knowledge of pressing decisions Agriculture about controversial natural resource and environmental issues. 2 THE ALLOCATION CHALLENGE he recent findings about the two different geologic regimes will KEY FINDINGS T allow planners to understand the dominant controls and streamflow, in both • Strong seasonal variations in the amount and quality of water delivered by trib- small and larger streams. Grant and his utaries to the Willamette River occur along an east-west gradient defined by the colleague, Dr. Christina Tague of the underlying geology and topography. In the winter, most water comes from the Department of Geography at San Diego western Cascades, while during the summer’s low flows, most water comes State University, have developed simple from the high Cascades. streamflow models that can be used to gain a reasonable prediction of future • The deep groundwater stored in the high Cascades emerges as springs fed by summer streamflow from early very large regional aquifers flowing through the young volcanic rock. This summer/spring streamflow measurements. water keeps flows steady even during drought years, and may have fallen as Using their methods allows a view of precipitation several decades ago. future flow even for streams without the pre-existing long-term gauge records upon • The correspondence between geologic controls and land ownership patterns in which these findings are based. the Cascades results in a very high proportion of streamflow in the Willamette originating on national forest lands. “In Oregon, competition for water in drought years between agricultural, urban, • The hydrologic and geologic context is crucial for planning water budgets, and habitat conservation has become a establishing stream temperature standards, and interpreting effects of land use controversial issue and understanding both and climate change on water resources of western Oregon. the pattern of flow regime and sources of water in larger streams is becoming increasingly important,” Grant says.