In the mid-1970s, this system was expanded with the construction of earthen channels called Lines E and E1 in the area between Brentwood, Oakley and Antioch.
Subsequent to the construction of the original regional facilities, the communities of Brentwood, Oakley and Antioch experienced tremendous urbanization, which continues to this day. With the conversion of agricultural lands to urban uses, the communities desired an improvement from a 50-year to a 100-year level of flood protection.
In 1994, the District amended the FCZ 1 plan to include a number of regional detention basins, the expansion of previously constructed reservoirs and widening of selected reaches of the Marsh Creek channel. The plan amendment also adjusted the legal boundary of the zone to better match the Marsh Creek watershed boundary.
In 2006, the FCZ 1 plan was modified a second time to change three smaller detention basins from regional to subregional facilities, and to transfer responsibility for these three basins to the City of Oakley.
In December 2007, the District published the Flood Control Zone 1 Financial Plan.