are interbedded with those from the nearby Los Angeles River as well as marine sediments left behind from ancient sea level changes. Prior to development of the floodplain, the river channels down the alluvial slope were poorly defined at best and frequently changed course with each winter storm, leaping back and forth between several different ocean outlets. In some years it joined with the Rio Hondo, which flows parallel to the San Gabriel and also passes through the Whittier Narrows, and flowed into the Los Angeles River; in others it would swing south toward either Alamitos Bay or Anaheim Bay, or even east towards the Santa Ana River. Once every few decades, a particularly intense storm would cause the rivers to burst their banks simultaneously, inundating the coastal plain in a continuous sheet of floodwater. The historical floodplain encompassed much of the San Gabriel Valley and a huge expanse of the Los Angeles Basin stretching from present-day Whittier to Seal Beach. The thick sediments of the lowland also trap an extensive local aquifer system. Historically, the aquifer was quite pressurized and close to the surface; natural artesian wells existed in many places. At the southern end of the San Gabriel Valley groundwater rose to the surface due to the damming effect of bedrock at the Whittier Narrows, and formed a perennial stream that ran across the coastal plain to the Pacific. In the 19th century irrigated agriculture was developed on a large scale in the San Gabriel Valley, and resulted in a severe decline of the water table as farmers drilled hundreds of wells. The San Gabriel Valley aquifer is now an important source of domestic and industrial water, and groundwater recharge operations are conducted using both local runoff from the San Gabriel River, and water imported through Los Angeles' aqueduct system. Ecology and environment The San Gabriel River once supported a rich lowland ecosystem on its broad floodplain, inundated multiple times each year by rain and snow melt. The result of this overflow was a network of riparian and wetland habitats, ranging from seasonally flooded areas in the north to alkali meadows (called "cienegas" by the Spanish), forests of willows, oaks and cottonwoods, and both fresh and salt water marshes in the south. At its mouth the river emptied into a broad estuary surrounded by thousands of acres of permanent marsh and swamp land, the result of a band