tidal action in the estuaries is generally restricted. 

 River runoff varies widely from year to year, and in 

 liigh discharge periods the fresh water of the estuary 

 expands but contracts again as discharge slows. 



Upstream discharge. The upstream watersheds of 

 all the rivers are less than 24,000 km^ (9,266 mi^). 

 Only the Sabine watershed drains an area greater than 

 10,000 km^ (3,861 mi^). Rain surplus is similar 

 throughout these watersheds, so discharge is propor- 

 tional to watershed area (fig. 3-19). The Chenier Plain 

 watersheds are small in comparison to that of the Mis- 

 sissippi River, which drains about 5 million km (1.9 

 million mi^ ) of upland surface. Since these watersheds 

 are small, their seasonal discharge patterns conform 

 closely to the local rainfall pattern, with only small 

 lag times. A typical seasonal discharge pattern for 

 Chenier Plain rivers is shown in figure 3-20. Here the 

 surface water input into the Calcasieu Basin at Kinder, 

 Louisiana, corresponds closely to the rain surplus in 

 the upstream watershed. 



The upstream discharge is important as a source 

 of sediments and nutrients and as a moderator of 

 salinity. Many organisms and processes are keyed to 

 the annual cycle of fresh water input. 



300 



<i 200 



• Sabliw 



Calcssieu 



• Mermentau 



5000 

 Watershed Ikm^ I 



Figure 3-19. The relationship of mean river discharge 

 to watershed area for three Chenier Plain 

 basins (U.S. Geological Survey 1977). 



3 



3 



70 



60 



50 



40 



30 



_20 



E10 

 o 

 - 



10 



Rain Surplus 



k 



t^ 



■L ^ 



J 



a> 

 O 



2.0^ 



o 



J? 



to 





JFMAMJJ ASOND 



Month 



Figure 3-20. Mean monthly rain surplus or deficit 

 estimated from rainfall, temperature, 

 and evapotranspiration (Borengasser 

 1977) for the Upper Calcasieu River 

 drainage basin, and the surface water 

 flow into the Calcasieu River at Kinder, 

 Louisiana (U.S. Geological Survey 

 1977). 

 Upstream inflow to coastal basins has, historically, 

 been modified by cultural activities. Modifications 

 range from indirect activities, such as clearing of for- 

 ests, which dramatically increases runoff (Likens and 

 Bomiann 1974), to control structures and dams that 

 directly modify flows. Within the Chenier Plain, the 

 watershed most seriously affected by discharge modi- 

 fication is the Sabine. The creation of the Toledo Bend 

 resevoir on the Sabine River (which carries most of 

 the water discharge into Sabine Lake) has resulted in 

 a large reduction in sediment load and total discharge, 

 and in a change of the seasonal timing of discharge. 

 This, in tum, has influenced salinity and biological 

 changes in the estuary (part 3.6.6). 



67 



