The Santa Clara River flows approximately 100 miles from its headwaters near Acton, California, to the Pacific Ocean, and is one of only two natural river systems remaining in Southern California. Flowing east to west through a beautiful valley formed between the Santa Susana Mountains and the Transverse Ranges, the river crosses lands with many uses and local governments. Threats to the ecological health of the river include urban development, channelization, oil spills, stormwater runoff pollution, and the possible resumption of large-scale aggregate mining in the channel. The river supports many populations: the human communities which dot its banks, plus a great variety of flora and fauna. Extensive patches of high-quality riparian habitat, totaling over 4,000 acres, are present along the entire length of the river. The Santa Clara River was selected by American Rivers as one of the nation's ten most endangered rivers for 2005.