Hydrology

Hydrology is the foremost determinant of vegetation species occurrence, NPP, biogeochemistry, floral and faunal habitat, and all floodplain functions and traits. Consequently, any insights into the nature of floodplain ecosystems and the basis of their societal value are predicated upon an understanding of hydrology. The 'flood-pulse' concept provides one framework within which to develop this understanding.

In this concept, the river and floodplain are considered as a single system and the 'rhythm of the pulse' (i.e., the hydroperiod) is the controlling mechanism which regulates exchange of energy and material between the river and floodplain. An influx of sediment and nutrients and export of organic carbon from the floodplain will occur at intervals dependent on the pulse rhythm. Examples of common rhythms include single, long duration and multiple, short duration which might be stereotypic of highorder river floodplains and low-order headwater streams, respectively.

In general, flood frequency and duration may decrease and increase, respectively, as stream order rises. When headwaters originate in mountainous terrain, narrow V-shaped valleys form and hydroperiods may be characterized as flashy (i.e., frequent flooding, with sharp rises and drops associated with stage levels). Hydroperiods reflect the integration of rainfall patterns, water storage capacities, and many other factors across the associated catchments. Consequently, stage level rises and falls are slower due to the 'buffering' that is provided by high storage capacities and the greater variability of other factors. Conversely, small catchments have much less storage capacity and, consequently, streams respond rapidly to precipitation events. As a result, floodplains of large rivers can stay flooded for significant portions of a year while low-order floodplains may be inundated frequently but for much shorter periods.

Interchange of water between floodplains and rivers is very complex and involves mutualistic influences. The nuances of those interactions form the basis of the role of floodplains as ecotones and regulators of energy and nutrient exchange. At low stage levels, water within swales and depressions may have originated with the river, precipitation, an upwelling of groundwater, or some combination. From a biogeochemical standpoint, the origin is significant in terms of the degree of spatial and temporal contact with the floodplain. At low stage levels, there is less opportunity for river water to contact the floodplain and, consequently, biogeochemical and dissolved organic carbon exchanges are minimal. As stage levels rise, the potential for the floodplain to influence the biogeochemistry of sheetflow increases as well. However, at some point, increasing floodwater volumes and higher velocities reduce contact with the floodplain. This is because a decreasing proportion of the sheetflow volume is in contact with the floodplain as volumes increase. Similarly, temporal contact is reduced as sheet-flow velocities rise.

There is also significant interaction between the river and floodplain in terms of groundwater. Channel waters often generate a head pressure which declines with distance from the stream bank. Groundwater transmittance will decline as hydraulic conductivity of alluvium decreases (e.g., clays have reduced conductance compared to sands). In humid regions, groundwater near the channel moves under pressure and will contact and mix with water that has seeped into the alluvium from adjacent uplands. As a result, groundwater mixing can be quite active during periods of low evapotranspiration.

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