Development of Water-Management System and Impact on the Hydrology of Southeastern Florida : Surface-Water Discharge

Since its inception, the canal conveyance system in southeastern Florida has had a major impact on both coastal ground-water hydrology and ecosystem ecology by increasing the flow of water from the Everglades to the Atlantic Ocean (Leach and others, 1972). This is evident in the long-term temporal variations in discharge and canal stage, which were assessed at selected surface-water sites near the coast and along the western edge of the urban-agricultural corridor (figs. 47-49). Mean monthly canal discharge and stage were averaged for the dry (November to April) and wet season (May to October) along the West Palm Beach, Hillsboro, North New River, and Miami Canals.

Observed variability in canal stage and canal flow prior to the 1960s is largely attributable to an incomplete drainage system under construction that responded directly to changes in the hydrologic system. Shallow, pre-1960s Everglades Agricultural Area canals that drained to major canals had limited capacity and were not yet regulated by control structures. Upstream areas tended to be overdrained during the wet season and lacked storage capacity during the dry season. Operational changes were put in place by the 1960s, helping to dampen or reduce high flow rates during high runoff events and to increase low flow rates during droughts. The seasonal dampening of discharge is evident in some hydrographs at coastal structures (figs. 48B, D, F).

Figure 47. Location of selected surface-water sites used to monitor canal stage and flow in southeastern Florida. [larger image]

Figure 48. Long-term mean monthly stage and discharge during the wet season (May-October) and dry season (November-April) at selected surface-water sites along the coastal reach of major canals in southeastern Florida. Site locations are shown in figure 47. [larger image]

Figure 49. Long-term mean monthly stage and discharge during the wet season (May-October) and dry season (November-April) at selected surface-water sites along the western margin and urban reach of major canals in southeastern Florida. Site locations are shown in figure 47. [larger image]

Long-term measured discharge along the coastal reach of major canals largely declined during the latter half of the 20th century (fig. 48B, D, F, H), whereas stage varied by season. Specifically, coastal canal stage levels (fig. 48A, C, E, G) were maintained mostly at higher levels during the dry season than during the wet season, presumably to help prevent urban flooding. Long-term wet- and dry-season canal stage along the coastal reach of the Miami River (fig. 48G) gradually increased between 1960 and 2000, possibly helping to minimize saltwater intrusion; less dramatic, but similar stage relations were exhibited along the coastal reach of the Hillsboro canal (fig. 48C). The long-term stage relations of the West Palm Beach and North New River Canals (fig. 48A, E) is more complex; stage was maintained at wet-season levels that gradually increased (May-October), but remained relatively stable or diminished slightly during the dry season (November-April).

Long-term flow and stage relations along the western margin and urban reach of major canals (fig. 49) are more difficult to explain than stage-discharge relations observed near the coast. Long-term flow into the urban area declined along the West Palm Beach and Miami Canals (fig. 49B, H), whereas Hillsboro and North New River Canal inflows (figs. 49D, F) were relatively consistent following emplacement of water-management controls. Conversely, long-term canal stage levels increased along the western urban reach of the Hillsboro and North New River Canals (figs. 49C, E), but decreased along the West Palm Beach and Miami Canals (figs. 49A, G).

The decrease in canal discharge within urban areas may be from municipal ground-water withdrawals that have helped to induce canal recharge. Withdrawals from the surficial aquifer system in Miami-Dade, Broward, and Palm Beach Counties increased from 85 to 770 Mgal/d in 1940 and 1995, respectively. Urban canal discharge decreases also may be due to the rerouting of surface-water flow to secondary canals, such as the LWDD (fig. 35). The rerouting is designed to elevate local coastal ground-water levels or recharge local well fields by canal seepage.