Aliso Creek is a small river in Orange County, California, in the United States. From its source in the Santa Ana Mountains to its mouth at the Pacific Ocean, it is about 19 miles (31 km) long. Its watershed is about 30.4 square miles (79 square kilometers) in size. The name comes from the Spanish word "aliso", meaning alder. Six tributaries join it along its length, which are Wood Canyon Creek, Sulphur Creek, the Dairy Fork, Aliso Hills Channel, Munger Creek and English Canyon.
The creek is heavily polluted. Urban runoff from streets in cities around the creek flow into it, contributing to an increased year-round flow and excessive pollution. To this day, the creek is still a big problem in terms of pollution at its mouth, the popular Aliso Creek Beach.