Water Quantity in the Mid-Coast Region
Water Quantity Snapshot
Water quantity and water quantity management in the Mid-Coast region was summarized during Step 2 of the planning process. The entire report on water quantity can be accessed here.
Streams in the Mid-Coast have high streamflow during the winter months (January-March) and low streamflow during the summer/Fall months (August-October) as a result of seasonal precipitation patterns.
Streams in the Mid-Coast are rain-dominated and responsive to precipitation, reaching high flows during rainstorms. Groundwater inputs maintain the base flows in streams during late summer and Fall months.
The Mid-Coast has eight active streamflow gage locations.
Information from river gages and water availability models help determine whether to issue new water rights. The water availability models take into account existing surface water and groundwater uses, and the amount of water available instream.
Generally, Mid-Coast groundwater is not very productive because of low permeability and low storage capacity of the regional rock formations.
All of the major river drainages in the Mid-Coast, with the exception of the Yachats River, originate at the crest of the Coast Range in Polk and Benton Counties and extend to the coast. There are eight major river drainages in the Mid-Coast: the Salmon River, Siletz Bay-Ocean Tributaries, Siletz River, Depoe Bay-Ocean Tributaries, Yaquina River, Beaver Creek-Ocean Tributaries, Alsea River, and Yachats River. Many streams in the Mid-Coast are ocean tributaries, meaning that they drain directly into the ocean rather than draining to a river, and are tidally influenced. The zone of tidal influence in these streams depends on the discharge of the stream and the type of tide.
Instream Water Rights
In 1987, Oregon legally recognized instream uses as beneficial uses of water, allowing for the creation of instream water rights that authorize the use of water instream to protect aquatic ecosystems, and out-of-stream water rights to be transferred instream. Instream water rights protect a specified amount of flow be kept instream within a certain reach or at a specific point along a stream. They have a priority date, place of use, and rate just like any other water right. Typically, instream water rights allocate specified flows for each month in the year.
Oregon has three “families” of instream water rights.
Instream water rights based on minimum perennial stream flows that OWRD administratively established in the 1950s and 1960s.
Instream water rights that state agencies, primarily the Oregon Department of Fish and Wildlife (ODFW),applied for after the passage of the Instream Water Rights Act, which have priority dates later than 1987 and are typically junior to many existing water rights. The beneficial use for these water rights is typically for maintaining flows for fish species, spawning, and migration. The Oregon Parks and Recreation Department (OPRD) and the Department of Environmental Quality (DEQ) are the other two agencies that can file for instream water rights for recreational purposes or pollution abatement.
Instream rights that have been created through transferring an out-of-stream water right instream (such as an irrigation water right) or through the Allocation of Conserved Water Program. These instream rights are typically for small amounts of flow (1 cubic foot per second [cfs]or less), but may have senior priority dates.
Precipitation and Streamflow Variability
Streamflow in the Mid-Coast is extremely variable. Topography, soil conditions, ground cover, forest cover, geology, and land use all influence the discharge and timing of streamflow. Urbanization affects the amount and timing of streamflow by decreasing the ability of the land to absorb rainfall and causing runoff to reach streams quicker. During individual precipitation events, streamflow can increase quickly and significantly in some streams. Streams also exhibit diurnal fluctuations - streamflow varies between daytime and nighttime as the amount of evapotranspiration increases and decreases.
Streamflow also varies on a daily basis as a result of precipitation events. Shortly after rain begins, streamflow begins to increase. There is lag time between when rainfall is greatest and when streamflow is greatest because it takes time for rain to reach the stream. Instantaneous peak flow is the maximum discharge (flow) at a given time. Peak flows can be much higher than the 80 percent exceedance flows, which are the flows that can be expected 80 percent of the time based on a base period of record. More information about peak flows can be found using the Peak Discharge Estimation Mapping Tool.