The Mosier watershed area covers approximately 49,725 acres. The watershed area encompasses all land drained by Mosier, Rock, and Rowena Creeks and their tributaries, all three of which discharge into the Columbia River. Associated lands which drain directly to the Columbia River are also included in the watershed area.  The Mosier watershed is located on the east slope and in the eastern foothills of the Cascade Range. Rowena Creek is a short, high gradient stream approximately five-miles long that flows west and then north from Oregon white oak-covered hills into the Columbia River near the community of Rowena (RM 182). This short stream drops approximately 2,000 feet in elevation from the headwaters to the mouth. Rowena Creek is bounded on the south by  Chenoweth Creek, on the east and north by the Columbia River, and on the west by Mosier Creek. Mosier Creek originates in the Mt. Hood National Forest near Gibson Prairie. It flows north out of mixed pine and fir forest and through fruit orchards, to enter the Columbia River at the town of Mosier (RM 176). The watershed is approximately ten miles long and two to eight miles wide. Elevation change from headwaters to mouth is approximately 3,300 feet. Mosier Creek is bounded on the south by Mill Creek, on the east by Chenoweth and Rowena Creeks, on the north by the Columbia River and on the west by Rock Creek and the Hood River sub-basin.  Rock Creek Originates in mixed pine and fir forest and flows north to enter the Columbia River at the town of Mosier (RM 176). Rock Creek watershed is a much wetter, more west-side ecotype than the Mosier and Rowena Creek watersheds. Rock Creek watershed is approximately seven miles long and one to two miles wide, and drops approximately 2,900 feet from headwaters to the mouth. It is bounded on the east by Mosier Creek, on the north by the Columbia River, and on the west and south by the Hood River basin. Mosier Watershed is covered by coniferous forests at elevations above 2,000 feet, giving wat gradually to oak forests at lower elevations. Vegetation patterns are determined principally by precipitation, which varies from 60 inches per year in the headwaters, to as little as 17 inches per ear on Sevenmile Hill.  Stream hydrology is influenced by winter precipitation and groundwater interactions. Persistent winter snowpack is limited to areas above approximately 3,000 feet, less than 15% of the watershed. Lower elevation, precipitation is a mixture of snow and rain, falling mostly in the winter. Groundwater provides a significant proportion of stream flow, particularly during low base flow periods in summer and late fall.