R.W. Matthews Dam and the Mad River-Our Source of High Quality Drinking Water
R. W. Matthews Dam forms Ruth Lake in southern Trinity County. It impounds runoff from the upper quarter of the Mad River basin, an area of approximately 121 square miles. Its capacity is48,030 acre-feet (AF).
A portion of the water stored in Ruth Lake is released each summer and fall to satisfy theHBMWD’s downstream diversion requirements, as well as maintain minimum bypass flow requirements in the Mad River below Essex. Although the HBMWD impounds water at Ruth Lake and diverts water at Essex, the operations do not significantly affect the natural flow regime in the Mad River. There are several reasons for this:
The total volume of water impounded and diverted by HBMWD represents a small percentage of the natural yield of the Mad River watershed. The Mad River’s average annual discharge into the Pacific Ocean is just over 1,000 ,000 acre-feet. Ruth Lake, in its entirety, represents less than 5% of the total average annual runoff from the Mad River basin. The entire 48,030 AFcapacity of Ruth Lake is not drawn down each year, so the amount of winter-season runoff captured in the reservoir is yet a smaller percentage of the total runoff. With respect to diversions, the current withdrawal rate at Essex is approximately 25 to 30 MGD (28,000 to34,000 acre-feet per year), which is only 3% of the total annual average runoff of the Mad River watershed. The full diversion capacity of 75 MGD (84,000 acre-feet per year) is just 8 % of the total annual average runoff of the watershed.
Tributaries downstream of Matthews Dam contribute significantly to, and are a major influence on, resulting flow rates in the Mad River. A former USGS gage station near Forest Glen was located nine miles below the dam prior to the confluence of any major tributaries. Annual mean flow at the Forest Glen gage station increased by an average of 22 percent compared to the mean flows just below Ruth Lake. The more significant tributaries on the Mad River are located downstream of this former gage station. These tributaries contribute significantly to Mad River discharge, and also provide a “buffering effect” during the few times the HBMWD is releasing from Ruth Lake less than the natural flow (e. g. during the first winter storms).
There is no out-of-basin transfer in the upper watershed, as occurs on some river systems. The water which HBMWD releases flows down the mainstem Mad River channel, and augments flows compared to what otherwise occurred naturally during the summer and fall. Flow augmentation has many beneficial effects, including expanding river habitat for the benefit of aquatic species. This benefit was addressed and acknowledged in HBMWD’s Habitat Conservation Plan for its Mad River Activities (HCP).
Click link below for HCP: