44 • Wetlands: Their Use and Regulation 



Figure 4.— Relationship Between Wetland Processes and Values 



Periodic inundation Wetland processes Ecological services 



C^^ Food and habitat 

 !_[]> Food chain support 



I ^ Floodpeak reduction 

 C^^Groundwater recharge 



t~~^\Na\p.r quality improvement 



r~^ Shoreline erosion control 



SOURCE: Office of Technology Assessment 



topographic depressions that retain runoff flowing 

 into them, at least until they are full. Also, during 

 flooding, the river overflows its banks and spreads 

 laterally across the flood plain , increasing its cross- 

 sectional area and conveyance capacity. By tem- 

 porarily storing storm water and providing capacity 

 to convey floodwaters, wetlands can reduce flood- 

 peaks and the frequency of flooding in downstream 

 areas. Vegetation in flood plain wetlands further 

 reduces the flow velocity of the river, thereby reduc- 

 ing potential floodpeaks in downstream areas and 

 riverbank erosion. If the soil in a wetland is un- 

 saturated, the soil itself will provide some storage 

 capacity during periods of flooding. While the value 

 of some wetlands for flood storage and conveyance 

 is well known, analytical techniques for predicting 



the magnitude of this service still are being devel- 

 oped. The value of inland wedands to reduce flood- 

 ing in downstream areas generally depends on the 

 area of the wetland, its location downstream, the 

 magnitude of flooding, and the degree of encroach- 

 ment on the wetland (16,31,67,88). 



Inflow-Outflow Measurements 



Only two studies were found that actually deter- 

 mined the storage capacity of a wedand during flood 

 conditions. One study measured water levels of a 

 cypress-tupelo swamp adjacent to the Cache River 

 in southern Illinois before and after flooding to cal- 

 culate the amount of flood water storage. The 90- 

 acre swamp, which is separated from the river by 



