The Umatilla River downstream of McKay Creek measures about 51 river miles in length. Through this reach, the river is intensively managed for irrigated agriculture. During the summer irrigation season, the hydrology of the river is largely influenced by a constant flow release from McKay Reservoir. These releases are removed downstream by irrigation diversions. A portion of these irrigation withdrawals are returned to the river as runoff or through groundwater percolation.
Discharge drops considerably and temperatures rise with the reduction in flow volume at the diversion points. Where irrigation drains enter the river, discharge increases moderately and often temperatures show a slight decline. In addition to McKay Creek, several other major tributaries join the Umatilla River in this reach including Birch Creek and Butter Creek, neither of which contributes substantially to the summer flow of the Umatilla River.
The primary factors limiting habitat quality throughout this reach are flow and temperature. Poor water quality, periodic passage problems, and channel straightening also negatively impact habitat quality and salmonid use. Because of the intensity of land use in this area, instream habitat diversity and riparian conditions are considered to be poor.
Seasonally inadequate streamflows persist throughout much of the lower Umatilla River. Low flows in the Umatilla River naturally coincide with periods of reduced precipitation and are compounded by irrigation withdrawals during summer months. The Umatilla Basin Project supplements flow to this historically dewatered reach. With the completion of Phase I and Phase II of the Umatilla Basin Project, target instream flows in the Umatilla River below McKay Creek (Table 33) can be met from September 16th to June 30th. Between the end of June and September 15, flows are limited in this section because the river is fully appropriated for irrigation withdrawals after the storage allocated for fish flows in McKay Reservoir are depleted. During July, August and much of September, low flow in the Umatilla River below Westland Dam continues to preclude habitat use in this area by all key species.
Table 33. Current Umatilla Basin Project target flows from McKay Creek to the Mouth of the Umatilla River (Bureau of Reclamation 1988).
Time of Year
Target Flow (cfs)
October 1 through November 15
November 16 through June 30
July 1 through September 15
September 16 through September 30
During irrigation season, the primary inflows are from irrigation return flows and drains, with the larger tributaries contributing little to the Umatilla River below Westland Dam. The irrigation withdrawals completely dewater much of the Umatilla River, resulting in an average daily flow over a 14-day period of less than 1 cfs (Table 34). The change in the low-flow statistics from Yoakum to the city of Umatilla demonstrates the impacts of irrigation on the lower portion of the Umatilla River. The one-year, one-day low flow at Yoakum is 129.8 cfs, versus 0.2 cfs near the city of Umatilla (Oregon Department of Environmental Quality 2000).
Table 34. Low-flow statistics for the Umatilla River below McKay Creek (Oregon Department of Environmental Quality 2000).
The Umatilla River downstream of McKay Creek is currently only used seasonally for migration and over wintering by key species (Table 35). Studies near Echo indicate this reach is only usable by bull trout during the months of November to early May (Bull Trout Working Group 1999). Historically, coho and fall chinook salmon used this reach for spawning and rearing, but native populations of both species are now extinct. The hatchery populations of coho and chinook have yet to establish sizeable natural populations and natural reproduction is limited for both species. At least one of the reasons for limited success with these species is because instream habitat quality in this reach has been compromised by insufficient flow, creating an environment unconducive to their full recovery.
Table 35. Key species season of use for the Umatilla River below McKay Creek