SU0013451 - Laserfiche WebLink

Home Browse Search

4.6 WATER RESOURCES Historically, Tracy Lakes and the Mokelumne River were connected during the wet winter and spring months when flows were high in the river and the lakes were full. In recent years, Tracy Lakes and the Mokelumne River have rarely been connected due to drought conditions and flood control structures such as Camanche Dam, which greatly minimize flooding in the lower reaches of the Mokelumne River. Additionally, levee's along the northern portion of the project site have been constructed to prevent flooding of the agricultural lands around the lakes from Dry Creek,another stream located approximately one mile north of Jahant Slough. Flooding Two primary sources of flooding influence the site, Jahant Slough locally and the Mokelumne River, regionally. Both of these watersheds have experienced numerous significant floods, including 1950, 1955, 1958, and 1986. The 1986 flood was triggered by the 50-year storm occurring in both Jahant Slough and the Mokelumne River watersheds. Jahant Slough and South Tracy Lakes have a watershed of 16 square miles. The 100-year peak discharge from this watershed is 858 cubic feet per second (cfs) (FEMA, 1987). Once lake levels have reached approximately 25 feet, water begins to spill from the lakes into the Mokelumne River. A USGS stream gauge is located on the Mokelumne River at Woodbridge,just upstream from the project site. This gauge has been collecting information on the river discharge since 1926. The greatest flood recorded on the river was a discharge of 27,000 cfs on November 22, 1950. However, since Camanche Dam was constructed in 1963, the frequency and magnitude of flooding in the area has been greatly reduced. The largest discharge was 5,340 cfs on March 8, 1986, after a large storm in mid-February of that year. The Federal Emergency Management Agency (FEMA) has mapped the flood plains of the Mokelumne River and Tracy Lakes. Most of the project and Tracy Lakes lie within the designated 100-year flood plain of the Mokelumne River(Figure 4.6-1). During the 100-year event it is expected that both the river channel and Tracy Lakes would be under one continuous body of water. The 100-year flood elevation is +29 feet (USGS datum)(FEMA, 1987). In the vicinity of the project site, the slope of the Mokelumne River begins to flatten as it approaches the Delta. During a 100-year flood event, floodflows spread out into the overbank areas and remain there until the flood state recedes. This overbank flooding serves to store water during the flood event, thus keeping flood elevations from further increasing. The lower Jahant Slough and Tracy Lakes area is within the 100-year floodplain and provides floodplain storage of Mokelumne River overflows. 1 4.6-2