GROUNDWATER DRAINAGE AS A CAUSE OF RIVER MEANDERING

Rivers exist, in part, to drain groundwater from adjoining aquifers. When the hydraulic surface of a meandering river is compared to the regional water table of the adjoining alluvial aquifer, an alternation is discovered wherein the river surface lies above the regional water table in places and below the regional water table in other places. This creates a surface water-groundwater exchange system very similar to that of a pool-and-riffle stream with infiltration occurring along the raised river reaches and outflow occurring along depressed reaches.

In this system, the downstream ends of the transverse river sections sit in the lowest position relative to the regional groundwater table and act as sumps draining the groundwater from the adjoining alluvial aquifer. The upstream ends of the transverse river sections sit in the highest position relative to the regional groundwater table and serve as potential sources of river water infiltration. Infiltrating river water, however, tends to carry mud into the pores of the riverbed sands thereby reducing the local permeability of the riverbed, whereas groundwater outflow tends to winnow fines from the riverbed sands thereby enhancing the local permeability. Thus, river water infiltration is minimized in the raised reaches, while groundwater drainage is maximized in the depressed reaches, thereby producing an efficient groundwater pumping system. In addition, besides the alternation of raised and depressed river reaches, the meandering river geometry spreads the sump points out laterally to more effectively capture the drainage from the adjoining alluvial aquifer.

Thus, river meandering can be viewed as a morphologic response to the need to efficiently drain the alluvial aquifer through which the river flows. As such, the width and sinuosity of the meander system depends primarily on the characteristics of the aquifer and on the water budget for the river-aquifer system, especially river volume and required drainage. Thus, flatter, wetter, finer-grained systems create more dramatic meandering than steeper, drier, coarser-grained systems.