Precipitation-Runoff and Streamflow-Routing Models for the ...

Study Area DescriptionThe Willamette River Basin (fig. 1) has an areaof approximately 11,500 mi 2 (square miles) and containsthe State’s four largest cities, Portland, Eugene,Salem, and Gresham. About 2 million people, representing69 percent of the State’s population (1990 census),live in the basin. The basin supports an economybased on agriculture, manufacturing, timber, and recreationand contains extensive fish and wildlife habitat.The Willamette River Basin has a temperatemarine climate characterized by dry summers and wetwinters. About 80 percent of the normal precipitationfalls between October and May. Mean annual precipitationranges from about 40 inches in the WillametteValley to 175 inches at crests in the Coast and CascadeRanges (fig. 4). About 35 percent of the precipitationfalls as snow at the 4,000 ft (foot) elevation, and morethan 75 percent falls as snow at 7,000 ft. Because thebasin is largely dominated by maritime air, bothannual and diurnal temperature ranges are relativelysmall. In the Willamette River Basin, the averageannual temperature ranges between 40 o F (degreesFahrenheit) and 65 o F (primarily dependent on elevation),with an average daily minimum of 30 o F in Januaryand an average daily maximum of 80 o F in July atlower elevations in the valley.The basin is bounded on the west by the CoastRange, on the east by the Cascade Range, on the southby the Calapooya Mountains, and on the north by theColumbia River. Elevations range from less than 10 ftabove sea level near the Columbia River to more than10,000 ft in the Cascade Range. The slopes and foothillsof the Cascade Range account for more than50 percent of the basin area. The Willamette Valley,generally considered the part of the basin below 500 ft,is about 30 miles wide and 117 miles long and representsabout 30 percent of the basin area. The mountainsof the Coast Range, reaching elevations of about4,000 ft, make up the remaining 20 percent of thebasin area. About 20 percent of the basin is above4,000 ft, which is considered the lower limit of thetransient snow zone.On the basis of physiography (Fenneman, 1931)and geology (Baldwin, 1981), the Willamette RiverBasin can be divided into three north-south-trendingprovinces: the Cascade Range, the Coast Range, andthe Willamette Valley. The Cascade Range is composedof volcanic rocks, consisting of (1) Tertiarybasaltic and andesitic rocks together with volcanicdebris, primarily in the western part of the range, and(2) Quaternary basaltic and andesitic lava flows, primarilyin the high Cascade Range. The Coast Range iscomposed of Tertiary marine sandstone, shale, andmudstone interbedded with volcanic basalt flows andvolcanic debris. Much of the terrain in the WillametteValley up to an elevation of about 400 ft is covered bysandy to silty terrace deposits that settled from waterponded in a great glaciofluvial lake (Glenn, 1965; Allison,1978). Alluvial deposits that border existing riversand form alluvial fans near river mouths werederived from the surrounding mountains, and theyconsist of intermingled layers of clay, silt, sand, andgravel.The main stem of the Willamette River is formedby the confluence of the Coast and Middle Forks nearEugene and flows 187 miles to the Columbia River.The main stem can be divided into four distinctreaches whose physical characteristics govern thehydraulics of flow. The upper reach extends fromEugene to Albany, river mile (RM) 187 to 119, and ischaracterized by a meandering and braided channelwith many islands and sloughs. The river is shallowand the bed is composed almost entirely of cobblesand gravel which, during the summer, are covered withbiological growth. The middle reach extends fromAlbany to the mouth of the Yamhill River,RM 119 to 55, and is characterized by a meanderingchannel deeply incised into the valley. The river isdeeper and has fewer gravel bars exposed in the summercompared to the upper reach. The Newberg Poolreach extends from RM 55 to Willamette Falls at RM26.5. Hydraulically, the deep, slow-moving pool canbe characterized as a reservoir. The pool is a depositionalarea for small gravel- to silt-sized material.Gravels are regularly mined from the streambed, andabout 6.5 million cubic yards have been removed overthe last 20 years (USACE, Portland District, NavigationSection, written commun., 1992). WillametteFalls is a 50-ft high natural falls; flashboards are usedto control pool elevation during summer low flow. Thetidal reach, from RM 26.5 to the mouth, is affected bytides and, during spring and early summer, by backwaterfrom the Columbia River. The tidal reach was notmodeled in this study.The Willamette River Basin has 11 major reservoirs(fig. 1); their combined usable capacity is nearly1.9 million acre-feet. The reservoirs are designed formultipurpose use, but their primary, legally designatedfunction is the maintenance of a minimum navigable5