FLOODING OF HISTORICAL BUILDINGS AT OREGON′S JESSIE M. HONEYMAN MEMORIAL STATE PARK LINKED TO INTER-DECADAL VARIATIONS IN PRECIPITATION

Historically high water levels of Cleawox Lake threaten to flood facilities at Jessie M. Honeyman Memorial State Park near Florence, Oregon. The Coos Bay dune sheet, which spans over 86 km of the coast, supplies sand to large, landward-advancing oblique dunes that impound several coastal streams forming a series of dune-dammed lakes, including Cleawox Lake. The Oregon Parks and Recreation Department commissioned an evaluation of natural and human processes possibly responsible for lake level rise, including historical precipitation, encroachment of coastal sand dunes, beaver dams, lake dredging, planting to control dune migration and park development.

Leveling surveys in 2008 indicate that the elevation of Cleawox Lake rose ~1.1 m since initial construction of park buildings by the Civilian Conservation Corps in 1937. High lake levels in 2008 appear to be linked to the cool/wet climate regime of the Pacific Decadal Oscillation (PDO) from correlations with long-term precipitation data. Annual and cumulative departure from average rainfall plots indicate that Cleawox Lake suffered nearly three decades of extreme drought in the early 20^th century prior to park development. Consequently, planning for park facilities in 1937 incorporated a lower than average lake elevation

Analyses of historical aerial photography document the encroachment of large oblique dunes that have pushed the lake shoreline northeastward over a period of 66 years, spanning 1939 to 2005. Peak rates of shoreline change (3.9 to 4.8 m/yr) occurred between 1972 and 1979. After 1984 the rate dropped below 1.5 m/yr. Over the entire 66-year period, the lake shoreline migrated northeastward at a rate of ~2.3 m/yr. Overlapping trends in the rate of shoreline change and decades-long patterns of climate fluctuation in the North Pacific suggest that shifts in wind regime attributed to the PDO may drive dune migration and partially control rates of shoreline change in Cleawox Lake.

Bathymetry data indicate that northeastward dune migration has partially filled the western part of the lake and promoted shoaling in the narrow north arm that leads to the lake outlet. Further impediments to stream flow through the outlet include increasing vegetation introduced to stabilize dunes and beaver dams, both processes that likely exacerbate lake level rise.