blind thrust fault which extends south from the Los Angeles Basin. The hills rose directly in the path of Aliso Creek, which cut a water gap through the range, forming Aliso Canyon. The uplift also diverted Sulphur Creek, which originally flowed south into Salt Creek, to turn north and join Aliso Creek. During the last glacial period (110,000 to 10,000 years ago), especially in the Wisconsinian glaciation (31,000 to 10,000 years ago), the climate of Southern California became periodically much wetter, with a climate similar to the present-day Pacific Northwest. At these times Aliso Creek was a river carrying much more water than it does today. During glacial periods sea level was as much as lower, increasing the stream gradient and thus its erosive force. These factors led to Aliso Creek carving out a much larger series of valleys than would appear possible with its present-day volume. As sea levels rose after the Wisconsinian glaciation, Aliso Canyon became a long narrow bay. Over thousands of years Aliso Creek filled in the bay with sediment, creating the flat alluvial valley floor seen today, while the creek itself remains as an underfit stream. Ecology Plants Before urbanization of the watershed, Aliso Creek and some of its tributaries supported a significant riparian zone dominated by native hardwoods such as coast live oak, sycamore, alder, cottonwood and arroyo willow. Many of the trees in Aliso Creek's riparian zone, especially near the mouth of the creek, were cut down in the Spanish Mission period to construct colonial settlements. During the early 20th century, groundwater withdrawal for agriculture killed off many of the remaining trees along the creek. The most significant remaining riparian habitat today occurs in Aliso and Wood Canyons, and along the uppermost headwaters of Aliso Creek. Increased erosion and pollution caused by urban runoff have had adverse impacts on the riparian zone. Invasive plants, including tobacco tree, castor bean, pampas grass, periwinkle, and artichoke thistle, but most notably the giant reed, have in many places replaced native trees. Giant reed was originally planted in the 1970s to control erosion. These invasive species are most prevalent along upper Sulphur Creek, the lower half of Aliso Creek, and Wood Canyon Creek. The hilly terrain of the watershed supports mostly grassland and coastal scrub vegetation. Native shrub species present in the watershed include California brittlebush, California buckwheat, California sagebrush, California goldenbush, coyote brush and mule fat.