During the cool, wet climate of the Pleistocene, a large lake formed east of the Cascades in southern Oregon and northeastern California. Known today as Glacial Lake Modoc, it had a surface area of 1100 square miles; its surface was 100 feet higher than that of Upper Klamath Lake, its largest remnant. Tule Lake and Lower Klamath Lake and their adjacent wetlands are also remnants of Lake Modoc, which extended eastward along the Lost River Valley and northward along the Wood, Williamson and Sprague Rivers that now feed Upper Klamath Lake.
Current geologic evidence suggests that the Upper Klamath Basin formed as a vast graben, divided by north-south trending fault-block ridges. Glacial meltwater from the Cascades balanced outflow through the Klamath River which carved a spectacular canyon through the Northern Coastal Ranges of California to reach the Pacific.
Toward the end of the Pleistocene, as the climate warmed, the mountain glaciers retreated, inflow to Lake Modoc diminished and the lake level fell, exposing vast wetlands between the remnant lakes. Of course, once European settlers colonized the region, wetlands were drained, rivers were dammed and tributaries were diverted for irrigation, producing the landscape that we find today. Fortunately, some recovery has occurred with the establishment of the Lower Klamath, Tule Lake and Clear Lake National Wildlife Refuges and spectacular congregations of migrant waterfowl and shorebirds still visit the Basin.