For some 10 miles, the Arroyo Seco River winds its way through a dramatic deep canyon. Its journey takes it over waterfalls and through stunning steep walled gorges with deep pools sculpted into the rock by the river's passage. In more gentle terrain, it meanders through the lush riparian habitat lining its banks. Steep, chaparral-cloaked slopes rise 1600' from the canyon bottom to the eastern boundary of the unit, the Arroyo Seco-Indians Road. While chaparral is the dominant plant community on the hillsides, broadleaf trees are found within the drainages of the small streams that flow into the river from the uplands of the addition.
This striking river contains remnant runs of endangered steelhead trout that spawn in its waters. The red-legged frog, another endangered species, is also known to inhabit the river.
