Ground-water resources of Snohomish County, Washington

Abstract

Snohomish County comprises an east-west strip, six townships wide, extending 60 miles from the eastern shore of Puget Sound to the drainage divide of the Cascade Mountains. Topographically, the eastern two-thirds of the county varies frown hills and low mountain spurs at the west to the continuous high, maturely carved mountains of the Cascade Range at the east. The western third of the county lies in the Puget Sound lowland section: it is made up largely of unconsolidated deposits, as contrasted with the hard rocks of the mountain section. High-level deposits of glacial debris in some places form a transitional ramp from the lowlands to the mountain topography; in other places the transition is abrupt. The principal rivers--the Snohomish, Skykomish, Stillaguamish, and Sauk--drain westward and northwestward to Puget Sound. The Puget Sound lowland, with its extensions up the river valleys, is economically the important part. of the county. Within that part., ground-water development is of particular importance. The climate is equable and dominantly oceanic, with an average of about 32 h. of rainfall annually, but with a pronounced dry season from June to September. A mean annual temperature of 52 F, a growing season of more than 200 days, and a variety of good soils form a setting in which supplemental irrigation can at least double the average crop production. Within the coastal lowland, plateau segments 200 to 600 ft or more in altitude are separated by flat-bottomed, alluviated river gorges. The river flats in some eases represent the surface of as much as 500 to 600 ft of glacial and alluvial deposits backfilled into canyonlike arms of the aneestral drainage system. The plateau segments are formed of the till-smoothed remnants of bedrock or the tabular segments of Pleistocene deposits. The Pleistocene deposits consist, above sea level, of about 200 ft of Admiralty clay and as much as 1,000 ft of deposits of the Vashon glaciation. The latter include as much as 300 ft of either clay or sand units of advance outwash, up to 150 ft of till, and variable thicknesses of outwash-terrace and train material. The Admiralty clay is composed largely of clayey materials without important quantities of ground water. Similar clayey sediments are known to continue downward for more than 1,000 ft below sea level. The sand unit and, to a lesser extent, the clay unit, are largely advance outwash of the Vashon glaciation. They are water bearing, and the position of their ground-water reservoirs--in flat-shaped bodies perched on the Admiralty clay beneath the plateau surfaces and slopes--makes them particularly susceptible to useful development. The till is a persistent ground-moraine deposit that mantles most of the area of the plateau segments and passes beneath much of the outwash and alluvium of the valleys. The till is a great vaster of precipitation; it. sheds off to the creeks much water that would otherwise recharge the ground-water reservoirs. A small amount of water percolates irregularly into and through the till or accumulates in the soil zones on top, where it is tapped by a great many 'hardpan' wells of small yield. Outwash terraces of gravels and sands, where they lie below the local water table, carry large quantities of ground water. The alluvial materials of the river valleys are good aquifers, but the water is iron bearing in many places and is saline in places in the lower parts of the Snohomish and Stillaguamish Valleys. At present, no good aquifers are known to exist in the bedrock formations. The metamorphic, granitic, and volcanic rocks of the mountainous parts of the county are largely nonpermeable. The Tertiary strata, which underlie the coastal lowland at depth and crop out about its inner margin, contain little or no water, and that which is present is usually of poor quality. The chemical quality of the ground waters is in general excellent. The iron- bearing water of the alluvial valleys is the princ