Ch. 3— Wetland Values and the Importance of Wetlands to Man • 47 



pared the rate of erosion of uplands buffered by 

 wetlands to that of unbuffered uplands. 



In a study of two similar sites on the Hacken- 

 sack River in New Jersey, the marsh vegetation at 

 one site was cut; at the other site, the marsh was 

 left in its natural condition (26). Both sites were 

 subjected to waves generated by heavy boat traf- 

 fic. While the uncut site exhibited only a negligi- 

 ble retreat of the bank over the year of monitor- 

 ing, the bank at the second site retreated nearly 2 

 meters, with most of the change occurring imme- 

 diately after the marsh was cut. 



In a second study, the rate of erosion of upland 

 areas at three sites on the Chesapeake Bay over a 

 20-year period was measured with aerial photo- 

 graphs. Wetlands eroded as fast as adjacent up- 

 lands; however, erosion of uplands buffered by the 

 wetlands was negligible (70). 



In a third study the retreat/ advance of the shore- 

 lines of an artificially planted marsh (Juncus roe- 

 merianus, Phragmkes australis, Typha latifolia, 

 and Spartina alterniflora) and of an adjacent un- 

 planted area were measured over a period of 8 years 

 (7). Initial erosion of the planted area was followed 

 by a period when the shoreline actively expanded 

 before it appeared to reach equilibrium. In general, 

 the volume of sediment eroded from the unplanted 

 shore averaged 2.3 m^ per lineal meter-year (m'/ 

 lineal m-yr.), nearly four times the average rate 

 observed in the planted marsh. In addition, the un- 

 planted shore retreated at a rate that was more than 

 twice that observed for the marsh-fringed shore. 



Limitations of Wetlands to Control Erosion 



Natural wetlands are typically found in low-en- 

 ergy environments, sheltered from extensive wave 

 action (4,17). Artificial wetlands, however, often are 

 constructed in higher wave-energy environments 

 where natural wetlands would not typically occur. 

 Young rooted plants are used rather than allow- 

 ing the shoreline to seed itself naturally. In addi- 

 tion, with many artificial plantings, a "toe" or low 

 ridge is constructed below the marsh to contain the 

 marsh soil and to reduce the impact of incoming 

 waves until the plants are established firmly. Most 

 of the literature citing the erosion-control functions 

 of wedands is based on observations of marshes spe- 

 cifically planted to control erosion. For example. 



in a 1981 survey of 86 marshes planted to control 

 shoreline erosion in 12 coastal States, 33 plantings 

 were found successful, 25 were partially successful, 

 and 28 failed (43). Even planted marshes, however, 

 were more frequently successful under less severe 

 wave environments. 



Ground Water Recharge 



Ground water recharge is the ability of a wedand 

 to supplement ground water through infiltration/ 

 percolation of surface water to the saturated zone 

 (88) . Some wetlands that are connected hydrolog- 

 ically to a ground water system do recharge ground 

 water supplies and assume an important local or 

 regional role in maintaining ground water levels. 

 However, owing to the low permeability of organic 

 soils or the relatively impermeable layers of clay 

 typically found in wedands, adjacent upland areas 

 often have a greater potential to recharge ground 

 water (16). In addition, wetlands may often serve 

 as discharge rather than recharge areas. ^ 



Ground water recharge can occur in isolated 

 (basin) wetlands, such as cypress swamps, prairie 

 potholes, Midwestern and Northeastern glaciated 

 wetlands, and flood plain wetlands. Cedarburg 

 Bog, adjacent to Milwaukee, Wis., is an example 

 of a high-value recharge area (58). Much of the 

 precipitation falling on this basin percolates down- 

 ward through the soil and enters openings in a dolo- 

 mite aquifer. Since the bog occupies the basin of 

 a former postglacial lake on a high point in the sur- 

 rounding topography, the water percolates radial- 

 ly away from the bog, influencing ground water 

 supply over an area of 165 mi^. 



While some wetlands may recharge ground 

 water, their recharge value relative to upland areas 

 may be low. In three watersheds in Minnesota, for 

 instance, the greatest amount of ground water re- 

 charge was found to occur on upland sands, and 

 the least in wetland peats (93). In addition, the 

 quantity of water recharged may vary widely. For 

 example, in one wetland studied only 39 gallons 

 per day (gal/d), or 0.05 percent of the annual water 

 budget, infiltrated the wetland (12). On the other 

 hand, the average yearly natural recharge calcu- 

 lated for Lawrence Swamp in Massachusetts was 



'Adamus and Stockwell, op. cit. 



