Scoggins Dam, Henry Hagg Lake, and related recreation facilities were initially operated and maintained by the Bureau of Reclamation. However, the Tualatin Valley Irrigation District is now responsible for the operation and maintenance of Scoggins Dam and the Patton Valley and Spring Hill pumping plants. When the reservoir is full, it covers 1,132 acres and stores approximately 59,950 acre-feet of water. Scoggins Dam was constructed to provide stored water for irrigation, Flood Control, municipal and industrial needs, recreation, fish & wildlife preservation, Water Quality Control and to provide drinking water for the cities of Forest Grove, Hillsboro and Cornelius. The recreation facilities are operated and maintained by Washington County. Henry Hagg Lake is one of Oregon's most popular recreational sites providing bike and hiking trails, fishing, water sports, and group picnic facilities.
The Tualatin Valley Irrigation District monitors and records agricultural water releases on an hourly basis at Scoggins Dam. These release numbers have been used to estimate TVID’s portion of agricultural water demand. It is important to note that the TVID releases more water that is required based on orders placed by its customers. The Tualatin Valley Irrigation District has historically over released water to assure that all peak TVID water diversions are met. Excess stored water from the TVID remains in the River and provides downstream flow augmentation as observed by river levels and on gauge readings at Elsner Bridge (river mile 13.7). The Tualatin Valley Irrigation District estimates that its over-release comprises about 20 percent of its total water use to cover peak demands. For planning purposes, agricultural water demand has been measured as TVID releases from Scoggins Dam.
Some actual consumption data (water consumed as opposed to water released) are available throughout the TVID system. Approximately seventy percent of the system is served off of a pressurized pipeline, that water is withdrawn at the Springhill Pump Plant. The remaining thirty percent of water use consists of withdrawals directly from the main stem of the Tualatin River. River withdrawals span the boundaries of the TVID service area, above Gaston at river mile 63.9 upstream, to Elsner Bridge. This part of TVID’s system is partially covered by primary natural flow rights, plus discharges from CWS’s Rock Creek wastewater treatment plant. Scoggins Dam releases provide supplemental water to natural flow water rights holders during the low flow period. The Oregon Water Resources Department Watermaster notifies these users when they are converted to stored Scoggins project water each year.
Irrigators who are customers of the TVID are exercising individual water rights to natural flows on the Tualatin River and its tributaries. The total amount of water withdrawn by these individuals is not metered and is therefore difficult to estimate. The OWRD maintains a database of individual water rights holders and points of diversion, but their actual water usage is not recorded. Non-TVID agricultural water usage has been estimated by considering the amount of irrigated acreage that is not served by TVID, and is therefore irrigated by natural flow.
Long term shifts in agricultural water consumption can be expected due to loss of farmland to urban development, from changes in crop types away from row crops to more water intensive nursery stock and from increased water conservation. Year to year variation in water demand can be expected based on weather, rainfall and agricultural commodity values.
More pictures of Scoggins Dam at the Tualatin Basin Watermaster website and the Oregon Water Resources Department website.