During operation of the spillway, materials suspended in the Mississippi River's water are deposited in the floodway and Lake Pontchartrain and Lake Borgne. In addition, the vast input of fresh water into these brackish and saline lakes has an immediate, short-term, adverse environmental effect.
The long-range effect, however, is extremely favorable because it simulates the natural flooding cycle of the river and provides a replenishment of valuable nutrients to the ecosystem. Spillway openings are strongly associated with increased oyster, crab and other fisheries production in Lake Pontchartrain and Lake Borgne for several years after the flood events.
With each opening, the river deposits an average of 9 million cubic yards of sediment, mostly silts and sand, within the floodway. These deposits are removed by private contractors and local government agencies for use as fill material in residential and industrial developments. This sediment is a valuable local resource since most of the surrounding region is near or below sea level.
In addition to the infrequent operation of the spillway for flood control, about every other year a small portion of the Mississippi River leaks through the spaces between the timbers of the spillway. This minor diversion of fresh water normally occurs for a few weeks in the spring or early summer when the river is high enough to exceed the elevations of the spillway weir but not high enough to warrant project operation. These minor diversions are termed leakage events (less than 10,000 cfs in comparison to a spillway opening with its design flow of 250,000 cfs).
The introduction of fresh water during leakage events simulates the natural cycle of overbank flooding and provides numerous ecosystem benefits to the aquatic and terrestrial resources in the spillway. These benefits include improved water circulation in the spillway's water bodies, nutrient introduction and restocking of fishery resources. Recreational crawfishing, for example, increases significantly due to the optimal conditions produced by these events. These frequent, small-scale diversions of Mississippi River water are also beneficial to the Lake Pontchartrain estuary.