Aliso Canyon is a canyon located in Orange County, California in the United States. The canyon is a water gap across the San Joaquin Hills carved out by Aliso Creek, possibly as recently as the last ice age. Located in a semi-arid climate, it supports a variety of plant communities – mainly chaparral and coastal sage scrub – and native animals, some endangered.
The canyon's northern divide once formed a formidable boundary between the indigenous Acjachemen and Tongva. Many Acjachemen archaeological sites have been found in the canyon. The fertile alluvial soil and grasslands of Aliso Canyon was used for orchards and grazing from Spanish conquest of the region until the early 20th century. Since the 1970s, the canyon has been the centerpiece of Aliso and Wood Canyons Wilderness Park.
Aliso Canyon has also suffered environmental damage in recent years. Upstream urban development since the 1950s has increased urban runoff into Aliso Creek, causing severe erosion throughout the valley floor. Pollution has also harmed the area's riparian zones. There have been plans to fix these problems, mainly by constructing drop structures to control erosion.
Description
Geography
Aliso Canyon is a massive gap through the south-central part of the San Joaquin Hills. The San Joaquin Hills are a small coastal mountain range in Orange County, about long and wide. The Aliso Canyon is about long measured from where it begins near Laguna Niguel Regional Park to its outlet at Laguna Beach. About deep on average and wide at its widest extent, it is one of the largest gaps through the San Joaquin Hills. Others include the Laguna Canyon and the San Juan Creek canyon.
The valley has three distinct sections: the first, upstream one is a broad and comparatively shallow stretch deep and wide; the end of this section is marked by a small summit to the south. The second longest stretch is wide, deep, and about long, stretching from the Wood Canyon Creek confluence to a wastewater treatment plant near the outlet to the Pacific Ocean. The last section of the canyon is a narrow, steep gorge wide, deep and about 1 mile (1.6 km) long. Most of the valley floor is a deep fill of alluvium that is up to thick in the deepest portions. From the valley floor, steep hills rise abruptly to the canyon rim in less than . These hills are dissected by many steep and short side canyons.
The major branch of Aliso Canyon is Wood Canyon, a -long, -deep gorge that runs generally southward to join Aliso Canyon about upstream of the ocean. Wood Canyon Creek drains Wood Canyon and joins Aliso Creek downstream of the beginning of the canyon. Cities bounding the canyon are Laguna Beach to the south and west, Laguna Niguel to the east, and Aliso Viejo to the northeast.
Geologic history
Despite its size, Aliso Canyon likely formed as recently as the Wisconsinian glaciation of the last glacial period, a length of time lasting from 60,000–10,000 years ago. During this time the San Joaquin Hills began to rise to their present-day form. (The uplift had actually begun as early as 1.22 million years ago, but the previous mountains had been eroded.) The Wisconsinian glaciation caused a shift in the entire climate of North America, pushing the cold, rainy climate of the Pacific Northwest south into California. The difference in sea level caused by the Wisconsinian glaciation was likely around - from the previous sea level, in the Sangamonian Satge, which was also about higher than the present-day sea level.
The new rainy climate of Southern California received around of rain each year, which caused the rivers and streams to become powerful and with enormous erosive force. Aliso Creek cut a V-shaped river valley through the San Joaquin Hills over this time span. Near the end of the Wisconsinian, the canyon was already a -deep gorge with a narrow, powerful watercourse at the bottom; the valley floor was likely around wide. When the Wisconsinian era ended, the glaciers melted and sea levels rose or so to their present level. Water backed up into Aliso Canyon and Wood Canyons, turning them into salt-water inlets. Over time, the creeks deposited enough sediment to fill the inlets with alluvium and create the present-day valley floor. This is the reason why the canyon resembles an aged U-shaped glacial valley instead of a relatively young V-shaped river valley.