The constantly changing mixture of freshwater and seawater results in an environment that is more extreme and variable than open ocean and freshwater systems. Numerous physical factors, such as the volume of river discharge, intensity of tidal action, distribution of tidal currents and elevations, compositions of sediments, and wind and wave energy contribute to the overall complexity of these coastal environments. The relationship between the tides and tidal currents may be obscured by a number of complicating factors, most notably the absence of confining banks or definite channels, the action of multiple progressive waves often moving in different directions, standing waves, frictional forces, and nontidal flows. Maximum velocity at flood tide approximates maximum velocity at ebb tide in the open ocean, but in estuaries having no complicating hydraulic factors, maximum flood- and ebb-tidal currents frequently differ due to the modification of the tidal wave in shallow water.