Horizon worked closely with the California Department of Fish and Wildlife (CDFW) and the State Water Resources Control Board (SWRCB) to update suction dredge regulations and develop a Subsequent EIR. CDFW regulates suction dredge mining for gold through a permitting program under Fish and Game Code Section 5653. In response to a lawsuit filed in 2005 by the Karuk Tribe of California, the court found that the 1994 EIR prepared for the prior program did not adequately disclose program impacts in light of new species listings and additional information that became available since 1994. Following the court mandate, Horizon was hired to prepare a Subsequent EIR as a basis to update the regulations.

Horizon developed a comprehensive statewide EIR that effectively updated suction dredge program regulations in tandem with the EIR development process. The project atmosphere was highly contentious, with disagreement among suction dredge miners and tribal and environmental interests. Horizon successfully led public meetings throughout the EIR process to obtain public input on the program and the CEQA process. Evaluating the effects of the program required statewide, integrated analyses of biological sciences, geomorphology, and water quality. The biological analysis considered program-related impacts to all listed aquatic species that occur in the state, which relied on the use of geographic information systems (GIS) in conjunction with database searches and an extensive literature review. Horizon also worked closely with SWRCB to ensure that all potential water quality impacts were considered and thoroughly evaluated. This water quality analysis included coordination with scientists from the U.S. Geological Survey and independent technical review by a distinguished group of scientists selected by the SWRCB.