nutritional value to animals that frequent the park. Giant reed in particular has invaded many riparian zones along Aliso Creek and reduced the biodiversity of these habitats. There are also numerous invasive animal species, including the Brown-headed cowbird, a brood parasite which lays its eggs in native birds' nests. Recent mitigation projects have utilized herbicides and trapping to reduce the population of invasive species within the canyon. Another issue affecting the park are hikers and mountain bikers who create unauthorized trails as shortcuts. Even before the opening of the park in 1990, motorcyclists would frequently trespass inside the boundaries, damaging sensitive habitat and slopes; in part because of this, motorized vehicles with the exception of maintenance and emergency workers are banned in the park. Rangers regularly block and re-plant unofficial trails, and visitors are urged to stay on the designated trails to prevent erosion. Erosion The Aliso Wastewater Management Agency (AWMA) was created in 1974 to reclaim domestic sewage for irrigation; a water treatment facility was established inside the future park as the overflow into Aliso Creek when the volume of reclaimed water exceeds demand in nearby cities. Since then, the population of south Orange County has grown faster than projected, increasing the volume of wastewater entering Aliso Creek and causing erosion problems inside the park. In the 1990s Orange County and the Mission Viejo Company constructed a dam on Aliso Creek inside the park, which was intended to control erosion and help restore riparian habitat. About 10,000 trees were planted to enhance habitat conditions. This project was known as the Aliso Creek Wildlife Habitat Enhancement Project (ACWHEP). However, after flooding damages in 1997-98 the dam was severely damaged, and has led to significant degradation of the river bed such that the creek now flows in a gully below the surrounding land. This has caused further environmental problems along the creek, and dried up former riparian habitat and wetlands. Further efforts to control erosion along the creek in the park, both to restore habitat and protect nearby waste-water pipelines, have been controversial as they would severely impact the existing habitat in the canyon. A 2008 proposal by the U.S. Army Corps of Engineers, would have "locked" the creek in place by building 20 drop structures and underground concrete walls to counter channel migration. The works would have involved moving as much as of earth and affected of the park. In