The upland area upon which Sammamish is located is underlain by a complex sequence of glacial and nonglacial deposits that overlie Tertiary bedrock. The depth to bedrock in the vicinity of the project area is approximately 1,000 feet, except at the southern end of Sammamish, where bedrock rises sharply and is exposed at the ground surface. This sharp rise in the bedrock surface results from movement along the Seattle Fault. The Seattle Fault is considered active and has recently been identified on the western shore of Lake Sammamish opposite the southern end of the City of Sammamish.
The Puget Lowlands have been glaciated six or more times in the past 2 million years. Each glacial advance may have deposited a sequence of glacial deposits and may have partially to completely eroded sediments deposited by previous glaciations or during interglacial periods. Outwash deposits from one or more glacial advances comprise the main aquifers at depth beneath the uplands.
The topography and near-surface geology of the Sammamish Plateau are largely the product of the last glaciation (known as the Vashon Stade of the Fraser glaciation), which receded from the area about 13,500 years ago. The troughs presently occupied by the Snoqualmie and Sammamish Rivers were created as glacial ice or subglacial streams cut into overridden deposits of previous glacial advances or intervening interglacial times. The ice sheet deposited glacial drift on the expansive upland plains. Drift in the Sammamish area consists principally of till, outwash sand and gravel, and fine-grained glaciolacustrine deposits.
The City of Sammamish is located in the eastern portion of the Puget Lowland on a broad, northerly-trending, upland plateau bounded to the east and west by the broad valleys of the Snoqualmie and Sammamish Rivers, respectively (Map 6, Appendix A). The surface of the upland lies generally between 300 and 500 feet above sea level and is bordered by steep bluffs along deep troughs occupied by larger rivers. The upland surface exhibits low topographic relief, with hills rising typically between 20 and 100 feet, and poorly drained stream valleys and local closed depressions. The floors of the Sammamish River and Snoqualmie River valleys are relatively flat with low valley bottom gradients.
The upland area is incised along the edges by small streams, with a few larger streams that extend into the interior and lie in valleys more deeply incised into the upland surface. Three such streams, Patterson Creek and Evans Creek, to the northeast, and North Fork Issaquah Creek, to the southeast, form the boundaries of the physiographic sub-area known as the Sammamish Plateau, which is largely occupied by the City of Sammamish. These drainages are bounded by steep slopes along the margins of the plateau and are incised approximately 150 to 300 feet below the upland surface. The Sammamish Plateau is bounded to the west by steep slopes that descend to Lake Sammamish, which lies at the south end of the Sammamish River valley and extends to depths as much as 100 feet.