The U.S. Army Corps of Engineers agreed to halt the project until it works out concerns with some environmental groups.
The Corps reclassified the wildlife preserve as a vegetation management area in 2010. Last month, workers bulldozed the area as part of a five-year plan to reduce crime and improve flood control.
"There's a lot of illicit activity occurring there in the area, as well as drug-use. From an operational standpoint, the woody vegetation and debris in the area caused a flood risk," said Tomas Beauchamp with the U.S Army Corps of Engineers.
The environmentalists are worried about ruining the natural habitat.
"This is 40-plus acres of natural habitat that has been cultivated and taken care of and represents native habitat in Southern California and the Valley, that has been flattened, that has been leveled. It is gone," said Dave Weeshoff with the Audubon Society.
The Corps said the vegetation will grow back and the area will remain safer.