EPHEMERAL GLACIAL LAKES IN SOUTHWESTERN ALASKA

Recently completed photo-geologic mapping shows that many parts of the Lake Clark, Lake Iliamna, Taylor Mountain, and Dillingham regions of southwest Alaska have been covered by ephemeral glacial lakes. These lakes appear associated with the waning stages of the glacial advances and range in age throughout the Quaternary. Evidence of glacial lakes is not common north of this area, whereas remnant lakes are common to the south. In the eastern part of the area, lakes were commonly damned behind terminal or recessional morainal ridges, similar to present-day Lake Iliamna and typically breach the moraine to drain. In one unusual case, a lake in the Koksetna River drainage, rather than breaching the moraine, drained through spillways to adjacent valleys in stages as glaciers in adjacent valleys receded.

In the western part of the study area, lakes were also damned by morainal ridges. Here, the Late Wisconsin glaciers did not extend far beyond the mountain front and therefore younger glacial lakes are restricted to mountain valleys, yielding the classic finger lakes of the Wood-Tikchik State Park. However, older more extensive lakes existed where morainal ridges or mountain masses served as barriers. Limited evidence suggests rapid draining of one of these lakes occurred north of Dillingham near Lake Beverley.

In the northwest part of the study area, south of Taylor Mountain, the lakes had a different form. Here, only small alpine glaciers, and possibly, a small ice-cap between the Nushagak and Mulchatna Rivers existed during early Quaternary glacial maxima. No lakes appear to have been directly associated with this ice. However, early Quaternary ice masses derived from the Alaska-Aleutian Range on the east and the Ahklun Mountains on the west coalesced into a single ice sheet in the middle and possibly lower reaches of the Nushagak River. Drainage from this ice sheet backed up in valleys north of the ice sheet and locally overtopped ridges. Where water flowed across divides, spectacular alluvial fans developed, providing clear evidence for flow into these former lakes.

In areas where morainal ridges or adjacent mountains did not provide containment, broad, low relief outwash plains developed; these are particularly apparent east of Dillingham draining to Bristol Bay and west of Taylor Mountain, draining toward the Kuskokwim River.