Devil's Gate Dam was the first of LA County's dams, built in 1920, long before requirements for analysis of environmental impacts. That didn't come until the California Environmental Quality Act took effect in 1970.
There has never a full official analysis of the impact of Devil's Gate Dam on fish or aquatic life, habitat and wildlife corridors, or stream impacts in Hahamongna or downstream of the dam. It's negative impacts on the Arroyo stream have been magnified by the construction of the flood channel south of the dam, which occured in the 1930s and 1940s.
The County's Draft EIR on the sediment program evaluates some of the impacts of their massive sediment trucking project, but not the overal impacts of Devil's Gate Dam or the County's flood facilities.