The Issaquah Valley Aquifer was formed by glacial activity during the Vashon Stade (stage)of the Fraser Glaciation The following Glacial History is reprinted with permission from the LOWER ISSAQUAH VALLEY WELLHEAD PROTECTION STUDY (Golder Associates November 1993)
3.1a2 Geologic History
Glacial ice entered the Puget Sound in late Pleistocene time (maximum extent about 15,000 years ago). The ice that occupied the Puget Sound area is known as the Puget Lobe of the Cordilleran Ice Sheet, which occupied northwestern North America in the early to late Pleistocene. Within the Issaquah area, only deposits of the final glaciation, known as the Vashon Glaciation, can be differentiated with certainty. The Vashon glacier originated in British Columbia, and flowed in a southern to southeastern direction through the Issaquah area (Curran, 1965). A pro-glacial lake of limited extent formed in front of the advancing glacier. Melt waters flowed south through channels east and west of Squak Mountain, depositing sand and gravel. As the glacier advanced, it modified the previously existing topography and deposited glacial till. Outwash sediments in front of the advancing glacier were often destroyed and reworked by the glacier. At its maximum extent, the glacier extended far south of Issaquah and may have been more than 3,000 feet thick in the Issaquah area.
The glacier is thought to have begun receding approximately 13,000 years ago. The recessional phase of the Vashon Glaciation is the most important to the geologic history of the lower Issaquah Valley. Based on evidence of a series of drainage channels, deltas, and ice-contact topography within the Issaquah area, Curran, 1965 reconstructed the recessional history of the Vashon glacier, which is discussed briefly below.
During recession of the Vashon Glacier, several episodes of ice stagnation occurred, which established stream drainages and associated depositional features and sediments. Curran (1965) recognizes seven periods of ice stagnation. Three of these stages are presented below. During these periods of ice stagnation, depositional features formed within the lower Issaquah Valley.
Booth (1990), recognizes five depositional stages summarized as follows:
Stage 1 (oldest): Consists of valley-wall and ice-contact sediments, located near the south end of Lake Sammamish, which were deposited when the glacier still occupied the Sammamish trough. Meltwater drainage was to the south along Issaquah Creek and Tibbetts Creek.
Stage 2: As the glacier receded farther north through present-day Issaquah, Glacial Lake Sammamish formed in what is now the lower Issaquah Valley. Melt waters flowed from the east along the North and East Forks of Issaquah Creek and deposited large deltas as they entered Glacial Lake Sammamish. Drainage out of Glacial Lake Sammamish at this time was still directed to the south through the Issaquah Gap and through Tibbetts Creek valley.
Stage 3: The glacier continued to recede, and meltwaters entering Glacial Lake Sammamish through the North Fork of Issaquah Creek, where a large delta formed. The outlet drainage of Glacial Lake Sammamish continued to shift to the northwest through the Cedar Grove, Kennydale, and Eastgate Channels (now occupied by I-90). The deltas along the eastern shore of Glacial Lake Sammamish continued to form at this time.
Stage 4: The glacier receded still farther, and the outlet drainage continued to shift farther to the north to the Inglewood channel. Streamflow through the eastern melt-water channels along the present North and East Forks of Issaquah Creek decreased substantially, as meltwaters began entering the lake from channels farther north. At this time, the lake occupied all of the present lake area and also the lower Issaquah Valley area.
Stage 5 (Youngest): This deposit consists of a low delta located just south of Issaquah occurring at elevations of between 100 and 150 feet above sea level, which formed during the last stage of glacial recession.
The glacier continued to recede until melt waters eventually ceased entering the lake, and the present drainage to the north was established. Lake Sammamish reduced in size to near its present configuration during this time.