cane barriers and similar structures can reduce the 

 salinity inflow, with commensurate changes in the 

 ecology of the area. 



Water diversion may disturb the migratory 

 patterns of fish. It has been found^* that the flow 

 reversal in the San Joaquin River, because of 

 exportation of water through a power plant, has 

 apparently affected salmon runs, presumably be- 

 cause "home stream" water was not present to 

 stimulate ascent and spawning. Further, the vigor 

 of estuarine circulation, greatly affected by flow, 

 determines the reproductive rate necessary for 

 maintenance of plankton populations.^ ^ 



Management authority is faced with the diffi- 

 cult task of weighing beneficial aspects against 

 harmful results in deciding on man-made changes 

 to an environment. 



^*D. Gaussle and D. W. KeUey, "The Effect of Flow 

 Reversal on Salmon," in ^«;i«a//?epo^?, Delta Fish and 

 Wildlife Protection Study, 1963. 



B. H. Ketchum, "Relation Between Circulation and 

 Planktonic Populations in Estuaries," Ecology, Vol. 35, 

 1954. 



An example of tradeoffs is the Bonnet Carre 

 Spillway constructed in 1932 to protect New 

 Orleans from Mississippi River floods. When 

 opened under flood conditions, it diverts water 

 into Lake Pontchartrain, and eventually the Gulf 

 of Mexico. All the receiving area is estuarine. In 

 Lake Pontchartrain, motile organisms are driven 

 out, and many non-motile forms are killed by low 

 salinities. Most or all oysters in mud covered beds 

 are destroyed, with lower loss over a wider area, 

 although oyster pests and predators are also killed. 

 Nutrient is added to the area in great quantities. 

 Following return to normal salinities, unusually 

 great production of shrimp and other marine life is 

 observed. Following a careful study^* it has been 

 submitted that the total beneficial economic effect 

 outweighs the partial oyster mortalities that occur 

 in some years. A photograph of this project is 

 shown in Figure 9. 



G. Gunter, Mortality of Oysters and Abundance of 

 Certain Associates as Related to Salinity, Ecology, Vol. 

 36, 1955. 



Figure 9. Bonnet Cane Spillway, Louisiana. Mississippi River floodwater is diverted into 

 saltwaters of Lake Ponchartrain. Here flood protection must be weighed against attendant 

 shellfish losses. (U.S. Army Corps of Engineers photo). 



III-41 



