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The ultimate detenninant of the sUUcture of floodplain ecosystems is the hydroperiod, or the timing (frequency and duration) and intensity of flooding. The hydroperiod. which is governed by the climate, soils, and geology of the area, determines the amount and move- ment of water in soils across the floodplain. This rise and fall of flowing water typically occurs at least once within the growing season. The saturation of soils for at least part of the year is one reason why wetlands tend to fonn in floodplains along stream channels. These hydrological features, combined with the connections to upland and aquatic ecosystems, are what make riparian ecosystems so special. (See Figure 7.) Soil~ and Nutrients - The distinctive attributes of soils in riparian ecosystems are directly influenced by the hydroperiod. which detennines the soil aeration (or oxygen level) as well as nutrients and content of organic material. In turn, the soil affects the structure and func- tion of plant conununities in these ecosystems. The aeration of soils is extremely important for rooted vegetation. When the corridor is flooded for long periods of time, low oxygen conditions can be created. Some planl~ have adaptations that help them to survive in such conditions. Soils in riparian areas (especially wetlands) generally have a high level ofnutri- ents because of the continual replenishment of nutrients during flooding. The periodic wetting of the soil also releases nutrients from the leaf litter. (See Figure 8, pagelO.) Vegewtion and Habi1at - Any ecosystem that fonns the edge of two other distinct ecosys- tems tends to be more biologically diverse than it' neighboring systems. This is indeed the case with floodplains, as nutrients. energy and water provide for high biological productiv- ity. The soil conditions that result from varying amounts of moisture in soils leads to a greater diversity of plant species in riparian areas. Floodplains may be characterized by different zones of vegetation, with shallow aquatic vegetation shifting gradually to shrubs and trees toward the upland elevations. This variety in plant life translates into greater diversity of habitats for wildlife. (See Figure 9, page II.) Diverse vegetation can support a wide variety of wildlife and smaller organisms that feed on the plants. In addition, the trees and shrubs of upland areas offer protection and nesting and roosting areas for many species. Trees standing or fallen adjacent to the Evapotranspiration .H\\; ...)-.... Illl ~'I ~ " ~ ~ I. "j ~ Precipitation Overland Flow & Runoff Fluctuating Water.2a~ -- ___--..!::!.ighWater~l_ Average Waler Level Recharge (Bank Storage) .... Low Water Level ------- ~ Figure 7 - Hydrologic Feature~' in the floodplain.