A. ALLUVIAL 



C. SPRING FED 



O'ND'J'FMAMJJAS' 



MONTH 



Figure 9. Hydrographs of four types of 

 southeastern floodplain rivers and 

 strearrs. 



Figure 10. Hydrographs (1974-75) of an 

 alluvial river (lower Apalachicola River, 

 FL) showing the possible effects of an 

 increase in floodplain width on water 

 levels, between upstream (solid line. 

 River Mile 126) and downstream (dashed 

 line. River Mile 68). (After Leitman 

 1978^ 



BLACKWATER RIVERS 



Blackwater rivers and tributary 

 streams originate in the Coastal Plain and 

 receive most of their discharge from local 

 precipitation. These streams have nar- 

 rower, less well-developed floodplains and 

 reduced sediment loads compared to those 

 of alluvial rivers.. The waters are rela- 

 tively clear, but highly colored (coffee- 

 colored) due to the presence of organics 

 (humic substances) derived from swamp 

 drainages. A hydrograph of a blackwater 

 stream (Figure SB) is characterized by 

 irregular discharge peaks that are due 

 almost wholly to frontal or local weather 

 events. Summer flooding, as well as more 

 typical winter-spring flooding, may result 

 from local storms. Unlike that of larger 

 alluvial streams (Figure 9A), the hydro- 

 graph of a smaller blackwater stream may 

 register dry periods during which dis- 

 charge may dwindle to near zero. 



Many blackwater streams are coastal 

 plain tributaries to alluvial rivers. 

 Water levels in some of these streams may 

 be controlled by the discharge levels in 

 the main river creating a "water dam" 

 effect (Wharton and Brinson 1979a). 



Ground-water seepage, or base flow, 

 is a particularly important component of 

 the discharge of blackwater streams. A 

 study (Winner and Simmons 1977) of a small 

 North Carolina Coastal Plain blackwater 

 stream (Creeping Swamp, N'C) (Figure 13) 

 resulted in a water budget in which over- 

 land runoff accounted for 17.75 cm or 6.99 

 inches (17%) and base flow runoff for 

 21.69 cm or 8.54 inches (20%) of the total 

 precipitation of 107.29 cm or 42.24 

 inches. Evapotranspiration accounted for 

 65.81 cm (25.91 inches) (61%) of the rain- 

 fall. A negligible 2% seeped underground 

 and was lost to the watershed. 



SPRING-FED STREAMS 



Spring-fed streams, characterized by 

 clear, alkaline flow issuing principally 

 from underground aquifers are common in 

 northwest Florida and in other areas 

 underlain by Tertiary limestone aquifers 

 (Figure 14). The discharge hydrograph of 

 a predominantly spring-fed stream (St. 

 Marks River, FL). (Figure 9C) is quite 

 flat compared to those of alluvial and 



17 



