well fields. In 1960 the con trol dam in the Biscayne Canal was moved from its original location, opposite the North Miami well field to a site two miles downstream. However, the move was too late and the field was contaminated by a massive slug of salt water. Recent samplings of water in that area indicate a slow freshening as a result of relocating the barrier. A boat lock and control dam have been proposed in the Miami Canal at 27th Avenue. This would furnish effective control for both the Miami and Tamiami Canals and give good protection to Miami's well field.
A perennial problem in the urban and agricultural expansion in Dade County is that of flood prevention during rainy seasons. Flood control has been and is being provided by a network of levees and drainage canals. Hdwever, the area must not be drained excessively; to do so would endanger the water supplies during prolonged drought. Many troublesome areas result where property of low elevations is developed without placement of sufficient fill to proper flood-control elevations. These areas must be protected against flooding. One such area is in North Miami Beach along the Oleta River.
Provision for adequate drainage in the North Miami Beach area necessitates deepening and widening the downstream reach of the Oleta River channel. The existing unimproved drainage condition approximates condition A in figure 2. The diagram, figure 2, shows the relative changes in-the position of the salt front in an aquifer within an area where a drainage system is being developed. Improvement of the Oleta River channel without a control dam would result in effective surficial drainage of the surrounding area, but also in a general lowering of ground-water levels, and an inland movement of salt water along the stream channel and in the Biscayne aquifer beneath and adjacent to the river. This condition would be comparable to position B in figure 2.
Prevention of excessive drainage and of coincident salt intrusion necessitates that a control dam be provided. Proper operation of the control