GLACIO-ISOSTATIC RESPONSE EAST OF THE CASCADES

Synglacial and postglacial tilting are evident in lidar topography of northeastern Washington and northern Idaho, substantiating Flint’s (1935) observation of “postglacial northward upwarping of the order of five feet per mile”. While mapping Cordilleran Ice Sheet flow lineations and ice-margin markers, I found relict late-glacial deltas that show former lake levels, which led to a search for related features that record tilting. Tilt azimuths are little constrained as observations are distributed along narrow valleys; tilts reported here are assumed N-up, towards the ice sheet.

At late-glacial Priest Lake, captured behind the terminal moraine of the Cordilleran Ice Sheet, relict tributary deltas and a relict spillway indicate postglacial tilt of ~1.4 m/km. In the Colville valley, relict tributary deltas built into recessional Lake Chewelah and its relict spillway indicate tilt of ~1.2 m/km. At Grand Coulee, heights of the Lake Columbia spillway, deltas built into the lake, and the top of lake-bottom sediments indicate tilt of ~1 m/km. Ambiguous morphology and likely diachroneity of some features suggest these tilts are perhaps accurate to +/- 15%.

Side-stream fans built into the SE end of Lake Chelan are little-incised, suggestive of a steady or rising lake level. In contrast, from Canoe Creek NW, side-stream fans are multi-story, with up to 20 m internal relief, indicating a falling lake level. NW- (or N-) up postglacial tilting of the lake is indicated.

Tilts observed east of the Cascades are similar to the 0.9 m/km N-up postglacial tilt evident in the Puget Sound area. The tilted domain likely included the Columbia River south of the ice margin, where the Columbia valley presently has a 0.6 m/km N-up gradient. Synglacial N-down tilting of the magnitude evident nearby would have reversed the valley slope with consequent infilling, a hypothesis supported by lake beds in the lower Wenatchee valley, the hanging outwash valley of Moses Coulee, and terraces at the mouths of Entiat, Corbaley, Swakane, Wenatchee, and Squilchuck valleys. Terrace elevations define a N-up slope of ~1.2 m/km. Linear extrapolation of this surface to the south suggests that the crest of the Cordilleran forebulge was somewhere south of 46°10’ N.