and sediment control. During the dry season the reservoir is often at a low level, in order to provide room for stormwater and allow county workers to remove built-up sediment from the basin. The northern part of the reservoir, when dry, is also used as the San Gabriel Canyon OHV area. There is no public boating access to either San Gabriel Reservoir or the downstream Morris Reservoir, which is used mainly for water supply. From World War II until the 1990s, Morris Reservoir was used by the U.S. Navy as a torpedo test site; the concrete launch ramp remains today and is easily seen from Highway 39, which runs through the canyon. San Gabriel Valley The river emerges from the San Gabriel Canyon at Azusa, a short distance below Morris Dam, where it reaches the wide and gently sloping alluvial plain of the San Gabriel Valley. At the mouth of the canyon the entire flow of the river, except during wet seasons, is diverted into the first of several spreading grounds that recharge the local San Gabriel Valley aquifer, an important source of local water supply. The usually dry riverbed then continues in a southwesterly direction, passing the ruins of the 1907 Puente Largo or "Great Bridge" that once carried Pacific Electric interurban trains, and under Interstate 210 into the flood control basin behind Santa Fe Dam. There are 17 drop structures or grade controls along this roughly stretch of river bed, to prevent erosion down the valley's relatively steep slope. Past the Santa Fe Dam – which when dry is used as the Santa Fe Dam Recreation Area – the river flows through Irwindale where it is the site of several major gravel quarries, which have operated in the San Gabriel Valley since the early 1900s to mine the rich alluvial sediments deposited by the river over millennia. From here Interstate 605, the San Gabriel River Freeway, parallels the river's east bank almost all the way to its mouth in Seal Beach. Throughout the San Gabriel Valley, the river flows mainly in an earth-bottomed channel between artificial concrete or riprap banks. Shortly below Interstate 10 at El Monte, the river is joined from the east by Walnut Creek, which restores a small perennial flow. Below this confluence it curves west and receives San Jose Creek, also from the east, before passing under SR 60. The river then enters the