Flood Awareness
WATERSHED DESCRIPTION AND FLOODING PROBLEMS Skagit County has abundant water resources with the Skagit, Samish, Sauk, and Cascade Rivers within its borders. These rivers have a history of flooding, causing extensive damages that affect the County’s economy, other resources, and its’ citizens way of life. This article describes the watershed area of the four rivers, their flood history, the typical flooding problems, and potential flooding causes. Localized flooding problems within specific areas of Skagit County are also discussed. Skagit County has within its boundary four major watersheds: the Skagit, Samish, Nooksack, and the Stillaguamish. The floodplains of the two latter watersheds are located outside the County’s boundary and will not be included in this plan. The Skagit and Samish basins comprise an area of 3,277 square miles between the crest of the Cascade Mountains and Puget Sound. Elevations within the Skagit and Samish drainage basins range from sea level at LaConner to 10,778 feet at the summit of Mount Baker. The northern end of the Skagit Basin extends 28 miles into British Columbia, where it borders the Frasier River Basin. The extremely rugged topography in the vicinity of Mount Baker gives way in the western part of the Skagit Basin to rolling country with a wide flat valley. Exclusive of the small area in Canada, the Skagit Basin has an area of 2,750 square miles. During major floods the Skagit River overflows a low divide between the Skagit and Samish River flood plains and the floodwaters from both streams intermingle on the Samish River flood plain. Flood problems of the two streams are, therefore, related, and both basins are generally treated as one large flood plain. The Skagit, third largest river in the western portion of the United States, flows southwesterly from its source high in the Cascade Mountains in Canada for 163 miles to tidewater in Skagit Bay, an arm of Puget Sound. It falls 1,600 feet in this distance, 1,300 feet from its source to Marblemount. The remaining 300 feet of fall are distributed over 92 miles in the lower basin. The river flows through a delta in two main channels, the North Fork and the South Fork, about 10 miles above the mouth, below Mount Vernon. These forks are nearly equal in length and during the usual range of river discharge the flow is so divided that about 60 percent is carried by the North Fork and 40 percent by the South Fork. The river is tidally influenced to the Great Northern Railway Bridge 15.4 miles above the mouth. The mean diurnal range of tide at the mouth is 11.1 feet and the extreme range is 19 feet. The three major tributaries augment the Skagit’s flow; the Cascade, which joins it near Marblemount; the Sauk near Rockport, and the Baker at Concrete. Several small watersheds are also tributary to the Skagit. These include the Illabot Creek, Finney Creek, Day Creek, and Nookachamps Creek watersheds. Many additional feeder streams also discharge directly into the Skagit River. Ross Dam Reservoir on the Skagit River controls the drainage from 978 square miles of watershed. It provides storage and head for a hydroelectric plant at the dam and supplements low flows for run-of-the-river hydroelectric plants at Diablo and Gorge Dams. Hydroelectric developments on the Baker River, a tributary to the lower Skagit River, include Lake Shannon controlling 270 square miles of watershed and Baker Lake, controlling an additional 215 square miles of watershed. A diversion system for supplying water to the City of Anacortes is located at Avon near Mount Vernon. The lower Samish River basin encompasses an extensive agricultural floodplain at the northern portion of the Skagit Delta with over 34,000 acres and 887 residences. For the purposes of this plan, Chuckanut Drive and Samish Bay generally bound the planning area on the north, on the east by Highway 99, on the south by Joe Leary Slough and on the west by Padilla Bay. These boundaries were established to coincide with the 100-year floodplain of the lower Samish River. The descriptions of the planning area characteristics in the following sections are based on information given in the Samish Watershed Characterization Preliminary Draft Report (Skagit County, 1994), which is part of the Watershed Action Plan. The lowland floodplains of the lower Samish River basin are defined by a vast network of drainage ditches, levees, and coastal dikes interspersed throughout farmland. Diking, filling, and channelization of the river and streams have significantly altered the natural lowland drainage. The whole area lies at nearly sea level. The upper watershed area, beyond the planning area boundaries, is composed of the foothills of the Cascade Mountains, ranges in elevation from less than 100 to greater than 4,000 feet. At one time, the Skagit River likely emptied into Samish and Padilla Bays through a network of braided channels (distributaries) fanning through the lowlands. Over time, these distributaries filled in and, as a result, the Samish became a separate river system from the Skagit. Within the past century, the surficial geologic conditions have changed as a result of the diking and dredging that has taken place in the lower Samish River basin. Wetlands and salt marshes have been filled, which has altered the natural course of streams. The lower Samish River basin is one of six sub-basins delineated in the Samish Watershed Characterization Report (Skagit County, 1994) for the purpose of identifying hydrologic units with similar characteristics within the Samish watershed. FEMA identifies virtually the entire lower Samish River basin as a 100-year floodplain. Significant flood events that occurred in 1983, 1986, and 1990 submerged large portions of the floodplain; the 1990 flood was considered a 35-year event. The Samish River drains about 139 square miles between the Skagit River Basin on the south and the Nooksack on the north. The Samish River originates on a low divide south of Acme in Whatcom County, and its tributary, Friday Creek, originates in the hills south of Bellingham. The river has a very narrow floodplain and much of its 20-mile length flows in a southwesterly direction between steep and rugged mountains. It outlets into Samish Bay, near Edison. The entire floor of the Skagit River Valley, the deltas of the Samish and Skagit Rivers, and reclaimed tidelands adjoining the Skagit and Samish River Basins comprise the floodplain. The floodplain covers 90,000 acres, including 68,000 acres of fertile land downstream and west of the city of Sedro- Woolley, and 22,000 acres of river bottom land east and upstream of this city. The valley upstream from Sedro-Woolley is narrow and relatively undeveloped, the agricultural area extending in general only to Concrete. Even in the reach from Sedro-Woolley to Concrete, about two-thirds of the bottomland is uncleared or is occupied by river channels and sloughs. The width of the floodplain varies from less than one mile along the tributaries and upper reaches of the main stem to over 20 miles in the lower reaches. Flat benches along the river characterize the upper flood plain, which are heavily covered with vegetation and sharply defined by steep canyon walls. Much of this area is unsuitable for farming because of the sandy, rocky soil and the changeable nature of the river channel in the steeper sections. Below Sedro-Woolley the valley drops almost to sea level and widens to a flat, fertile outwash plain adjoining the Samish Valley to the north. These fertile lands are ideal for farming. The outwash plain extends west through Mount Vernon to LaConner and south to the floodplain of the Stillaguamish River. Potential flood damage in the Skagit River Basin is greatest in the flood plain. The flood plain is primarily agricultural, but includes a large proportion of the county’s urban and rural population, many manufacturing plants, and major transportation routes. Runoff from the Skagit River basin depends on rainfall and snowmelt as provided by climatic conditions. Due to the proximity of the Pacific Ocean to the Skagit Basin, the influence of maritime air masses is pronounced in both the temperature and precipitation regimes, producing a mild but wet climate. During the winter, the Skagit Basin, lying directly in the storm path of cyclonic disturbances from the Pacific Ocean, is subject to numerous storms, which are frequently quite severe and may follow one another in quick succession. On the mountain slopes, storm precipitation is heavy and almost continuous as a result of combined frontal and orographic effects. During summer months, the weather is warm and relatively dry as the Aleutian low-pressure system is displaced by a semi-permanent high-pressure system. The Skagit River Basin is subject to winter rain floods and annual high water due to snowmelt runoff. Low flows occur during August and September after the snowpack has melted and the ground water flow has been partially depleted. A summary of streamflow data for the key stream gauges is shown in Table 1. TABLE 1. Streamflow Data – Skagit River Basin*
*Based on records of the U.S. Geological Survey through September 1994. 1Incomplete information due to gauge damage. |
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