- Harney County)
(From Atlas of Oregon Lakes, Johnson et al. 1985). Cottonwood Reservoir is a small irrigation impoundment in the drainage of the Middle Fork of the Malheur River in eastern Oregon. It was formed in the mid-1960s with the construction of a 73-foot high earthfill dam on Cottonwood Creek. It is reported to have a capacity of 3700 acre-feet, but when mapped in August 1982 it was more than 20 feet below full pool and held less than half this volume. Irrigation storage is the primary use of the water, thus the water level drops considerably through the summer as withdrawals are made and delivered to irrigated agricultural land downstream on Cottonwood Creek.
The headwaters of Cottonwood Creek are in forested land within the Malheur National Forest, while in the lower reaches is land administered by the Bureau of Land Management. The reservoir itself is entirely on B.L.M. land. Fish have grown well in Cottonwood Reservoir since it was first stocked in the 1960s; however, there has been relatively little angling activity. No facilities have been provided for visitors, although improvements are planned.
Cottonwood Reservoir is eutrophic; it has high concentrations of major ions and total phosphorus. The bottom of the reservoir is mud, and wave action sometimes gives rise to turbid water. Water transparency is reduced by both the suspended sediment and an abundance of phytoplankton (Aphanizomenon flos-aquae) blooms. In 1982, the reservoir was stratified as early as June 23, with the thermocline about 16 feet (5 meters) deep. Dissolved oxygen in the hypolimnion was near saturation levels; however, lower dissolved oxygen concentrations would be expected later in the summer. Water levels fluctuate greatly, and this prevents establishment of macrophytes.