STREAMBANK EROSION RATES ON THE LOWER ST JOE AND COEUR D'ALENE RIVERS, ID

Natural levees on the lower portion of the St Joe and Coeur d'Alene River deltas are building into Coeur d'Alene Lake primarily by crevasse splay deposits and vertical accretion from overbank floods that averaged 0.9 cm/yr above the 1980 Mt St Helen's ash layer on the St Joe River. Erosion and reduction of the lower ends of the levees has occurred primarily by inundation and boat wave erosion since the Post Falls Hydroelectric Project (HED) was built in 1906.

Erosion rates along the levees were measured using a series of rectified historical aerial photographs and 1908 map, with 1 year of data from bank erosion pins, from in-place former riverbank tree stumps, and by the width of the prominent erosion notch formed at the Lake level that is controlled during the summer boating season by Post Falls HED¹. Measured rates of erosion were comparable between the methods.

Geometric control to rectify the series of aerial photographs and 1908 map was based on a 2003 digital orthophoto mosaic taken at known lake level. Ground survey showed accuracy was good because of the high contrast of land and water, limited overhanging vegetation, common elevation of the water level that provided numerous control points, and the quality of the base orthophoto used for geometric control.

Width of the boat wave formed erosion notch at the summer lake level was measured based on GIS analysis using a 1 ft contour side scan sonar survey and photogrammetric derived water line at known lake level, and provided erosion rate estimates over the full length of the lower rivers.

Bank erosion along the downstream 13 km of the St Joe River averaged 6 to 9 cm/yr and was greater than upstream because it combined waves from heavy boat traffic, wind waves, flood currents, freeze thaw, and rain splash erosion. Further upstream erosion was only on the inside of the levees and varied from 3 to 6 cm/yr. On the Coeur d'Alene River average erosion rates were lower, 1.5 to 3 cm/yr, with similar heavy boat traffic, and is attributed to the increased density and shear strength of portions of the banks because of metal rich mine wastes in the levee deposits. Erosion pins and boat counts indicate boat waves were the primary energy source for erosion and showed an average 50% of bank erosion occurred during the 3-month summer boating season.

¹http://www.avistautilities.com/resources/relicensing/spokane/documents.asp?DocID=2004-0046