Resumen de Channel responses to declining flow on the Rio Grande between Ft. Quitman and Presidio, Texas
A profound decline in the discharge of the upper Rio Grande following upstream water development set in motion a chain of responses in channel geometry and sediment budget. The case of the Rio Grande provides a well-documented example of the evolution of channel geometry in response to change in a single external variable.
In a regime of declining flow, channel change was driven by an excess of sediment relative to the energy available to do work in the stream channel. Several different responses can be identified, which operated at different rates. Responses can be ranked according to reaction time, to define 3 stages in channel evolution. Stage I is identified as the shrinking of the channel as excess sediment was deposited within it. During Stage II channel capacity began to stabilize as overbank flow carried sediment onto natural levees and floodplain. Valley-wide aggradation began, and avulsive migration resumed, especially at the mouths of tributaries. Where Stage III is occurring, lag deposits of bedload at tributary mouths are locally regrading the valley.
Initial shrinking of the channel produced positive feedback to the decrease in discharge and sediment transport capacity. Not until later stages did subordinate responses appear which tended to balance the sediment budget, and these involved lateral migration. The river approached a balanced sediment budget both by increasing its capacity by shortening and steepening, and by decreasing sediment inout by migrating away from tributaries.
