by the County of Orange and the Mission Viejo Company, and intended to rehabilitate of former riparian areas that were dried up due to downcutting of the stream channel. A high concrete dam was built about downstream of Aliso Creek Road, inside Aliso Canyon, to increase the water level so that the riparian area could be restored. Due to poor design as well as storm damage from 1998 flooding, the dam ended up causing further erosion of the creek downstream due to the rapid flow of water cascading over the structure, and the project was abandoned. The dam is now considered a failure risk and must be periodically grouted to maintain its stability. In 2006 the U.S. Army Corps of Engineers began studies for the Aliso Creek Mainstem Ecosystem Restoration Project, which aims to control erosion and restore riparian habitat along the lower of the creek downstream of Pacific Park Drive, as well as short stretches of lower Wood Canyon and Sulphur Creeks. The project would remove the existing drop structures and barriers along this section of the creek and construct 47 low rock riffles to maintain a constant gradient. Parts of the creek prone to erosion would be realigned to slow down the water velocity, and the creek banks in several places would be stabilized with buried concrete walls. A total of of riparian areas would be reconnected to the creek's floodplain. All non-native plant species would be removed. As of 2017 this project had an estimated cost of $91 to $99 million. The USACE project has been criticized for its potential large impacts on Aliso and Wood Canyons Wilderness Park as it would require extensive re-grading of the stream channel and banks, with of excavations and more than of fill. In addition, many of the proposed structures are intended to protect the sewer lines along Aliso Creek in Aliso Canyon, rather than for environmental restoration. The Sierra Club has advocated for the closure of the Coastal Treatment Plant in Aliso Canyon, and for wastewater to be recycled or treated at the nearby J.B. Latham plant instead, which would significantly reduce the cost of the Aliso Creek restoration project and reduce its environmental impacts. As of 2016, the Latham plant was operating at about 50 percent of its capacity. A number of artificial wetlands have been constructed in the Aliso Creek watershed to replace historic wetlands lost to urban