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. , ALASKA ... -, " I' 'o4J o 250 500 MILES 6 2~ 500 KI~OMETERS /v~~\ 1\:;-';" "'~ r.....:t. :it ~ ,..---..... .. r~./'." ~;ff. ~ _~~..7 ~ .t,,{e( -, ' ;' '. /." ~ \-, o I o 200 MILES , 200 KILOMETERS HAWAII WETLANDS This map shows the approximate distribution of large wetlands in the Nation. Because of limitations of scale and source material, some wetlands are not shown . Predomin(lnlly wetland ~ Area typified by a high density of small wetlands .C'" '= '0~'" o 100 MILES \ \0 f----r--l 1 ..... o 100 KILOMETERS Figure 1S, Wet/ands are present throughout the Nation, but they cover the largest areas in the glacial terrain of the north-central United States, coastal terrain along the At/antic mid gulf coasts, mid riverine terrain in the lower Mississippi River Valley, A major difference between lakes and wetlands, with respect to their interaction with ground water, is the ease with which water moves through their beds, Lakes commonly are shallow around their perimeter where waves can remove fine-grained sediments, permitting the surface water and ground water to interact freely, In wetlands, on the other hand, if fine-grained and highly decomposed organic sediments are present near the wetland edge, the transfer of water and solutes between ground water and surface water is likely to be much slower, Another difference in the interaction between ground water and surface water in wetlands compared to lakes is determined by rooted vegetation in wetlands, The fibrous root mat in wetland soils is highly conductive to water flow; therefore, water uptake by roots of emergent plants results in significant interchange between surface water and pore water of wetland sedi- ments, The water exchanges in this upper soil zone even if exchange between surface water and ground water is restricted at the base of the wetland sediments, 21