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<br />. <br /> <br />2.2 Soils/Geomorphology <br /> <br />The valley floor ranges from approximately 500 to 1,000 feet in width throughout the MA. The <br />river channel averages about 30 feet in width, but varies dramatically in character from a <br />meandering, single-thread channel to a wide, braided channel. Between the valley walls, the <br />valley floor consists of several relatively flat surfaces underlain by alluvial sedimentary units that <br />have been deposited by the river and its tributaries. These sedimentary deposits and their <br />associated fluvial geomorphic surfaces provide the physical substrate for the presence of <br />wetland/riparian plant communities of the valley floor. <br /> <br />Within the MA, four alluvial surfaces are present: I) the channel-bar subzone, 2) the floodplain <br />subzone, 3) the low terrace subzone, and 4) the high terrace-alluvial fan surface (Figure 5). Only <br />the first three ofthese are within the river's zone-of-influence. The area of most active fluvial <br />processes is the channel-bar subzone. This area incorporates the river channel and includes <br />recently deposited sediment on either one or both riverbanks. The soils are typically poorly <br />developed pebbly sands and contain little organic matter. This area is subject to frequent <br />flooding and is prone to rapid erosion, especially along unvegetated portions of riverbanks that <br />lack stabilizing rootmasses. The floodplain subzone is located about 2-feet above the channel- <br />bar subzone. This is a predominantly depositional environment, with clays, silts, and sands <br />deposited from suspension during flooding. Soils are finer grained than on the channel-bar <br />surface, but still may be quite pebbly in places. This area is subject to frequent flooding and is <br />also prone to rapid erosion, particularly if vegetation has been removed. Most of the existing <br />wetland/riparian vegetation within the MA.is found on the channel bar and floodplain subzones. <br /> <br />Figure 5. Cross-section ofthe La Plata River valley. <br /> <br /> <br />Zone of Influence <br /> <br />Located 3- to 6-feet above floodplain level is the low terrace surface. This fluvial surface was <br />the floodplain prior to turn-of-the century channel incision that created the current floodplain. <br /> <br />-12- <br />