The Lake Oswego City Watershed has a significant impact on Oswego Lake water quality. The seven square mile watershed is completely contained within the Lake Oswego city limits. Lake Oswego Corporation works cooperatively with the City of Lake Oswego as stewards of the watershed since the lake is the iconic landmark of the community.
From the Atlas of Oregon Lakes (Johnson et al., 1985)
"Lake Oswego has an interesting geologic history. It occupies a former channel of the Tualatin River, carved in Columbia River basalt. Downcutting and excavation eventually captured the main channel of the Tualatin River, leaving the old Lake Oswego route abandoned. About 13,000 years ago flood waters from glacial Lake Missoula raced down the Columbia River, backed up the Willamette River, and poured through the Lake Oswego gap, deeply scouring the enlarging the channel."
Initially the only watershed for the lake was the surrounding 4500 acres (Fig. 1). In 1872 a canal dug out of solid rock was completed between the Tualatin River and Oswego Lake. That year also marked the completion of the first dam that raised lake level several feet.
This navigation canal exposed Oswego Lake to the 455,000 acre Tualatin River watershed ( Fig. 2). Currently an electronically controlled headgate regulates the amount of water allowed into the lake from the Tualatin River.
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