Historically, no other drainage districts in the area have used the SLD for the conveyance of drainage water. The Grassland Basin Drainers (GBD) discharge drainage water through a more circuitous route, utilizing the wetland water supply channels, Salt Slough and Mud Slough, for disposal of the drainage water to the San Joaquin River upstream of its confluence with the Merced River. Agricultural return flows, including GBP drainage water, dominates the flow in the San Joaquin upstream of the Merced for most months of the year.
Since 1985, when wetlands ceased to use water with selenium concentrations greater than 2ppb, wetland managers have used a complicated "flip-flop" system to alternately transport agricultural drainage and wetland supply water through the Grasslands conveyance system. This system requires a high level of coordinated water management and requires that channels be flushed of selenium contaminated drainage water before being returned to conveying wetland supply water. This results in inefficient water use, and the potential for contaminating wetland water supplies with drainage water during this "flip-flop" operation. In addition, scheduling restrictions inherent in this system restrict, and sometimes prevent, wetland managers from utilizing otherwise available water supplies to optimize habitat and wildlife benefit. The passage of the Central Valley Project Improvement Act (CVPIA) has recently required that an increased volume of fresh water be allocated to wetlands. These increased allocations have made the development of a program which allows for the more efficient supply of freshwater to the managed wetlands all the more crucial.