Hydrodynamic and geomorphologic dispersion: scale effects in the Illinois River Basin

The objective of this work is to determine the relative effects of hydrodynamic and geomorphologic dispersion on the hydrological response of the Illinois River Basin (IRB) as scale increases. The specific hypothesis that was tested is that as basin size increases, the river network structure, as compared to channel hydrodynamic properties, plays an increasingly dominant role in determining the hydrologic response. The analysis was performed on eight of the major watersheds in the IRB in order to provide an adequate representation of basins that contain streams of order six or greater. The basins studied include the Des Plaines, Mackinaw, Vermilion, Fox, La Moine, Spoon, Kankakee, and the Sangamon, and have magnitudes ranging from order six to order eight. The geometric and hydrodynamic properties were derived from the analysis of digital elevation model data and from the hydraulic geometry equations for various subcatchments of the IRB put forth by Stall and Fok [Univ. of Ill. Water Res. Center Res. Rep. 15, 1968]. The hydrodynamic and geomorphologic dispersion coefficients were determined for each order stream of the eight basins and for constant flow frequencies, then compared. The results contradict the original hypothesis, for at small scales, geomorphologic dispersion tends to dominate, the extent of which depends upon the flow frequency, and at large scales, geomorphologic dispersion is less dominant. This occurs because of the behavior of the path lengths of a stream network, which geomorphologic dispersion depends upon. In addition, at high flow frequencies the geomorphologic dispersion dominates, and at low frequencies the hydrodynamic dispersion begins to play an increasingly important role, although the geomorphologic dispersion still dominates. This dominance suggests that the geomorphologic parameters of a watershed could be more important in characterizing the hydrologic response of a river basin than hydrodynamic parameters.