New Mexico Geological Society Annual Spring Meeting — Abstracts

Schumm (1985) proposed that the perceived magnitude of a given geomorphic event decreased when considered over longer intervals of
time. Thus a perceived megaevent at one time scale would become a mesoevent, microevent, and nonevent at longer time frames. While most geologists are concerned with time scales of 10⁶ to 10⁸ years, many fluvial geomorphic adjustments take place over 10^-2 to 10² years. Monitoring two reaches of the Rio Puerco during the past decade, coupled with earlier observations and aerial photographs, demonstrates that the ephemeral stream makes rapid at-a-station adjustments to major changes in slope, depth and width along its channel. The adjustments are dependent on the magnitudes and sequence of post-event flows in the channel.

In the first reach, during early spring runoff in 1988 a meander loop was cut off across its neck, an event that occurs in this area perhaps once per century. Gradient around the meander loop was about 0.003 (ft/ft; m/m) whereas the gradient across the neck was about 0.25 (1.7 m in less than 7 m). Within two months, the stream channel had adjusted its gradient to about 0.01, affecting at least 555 ft (170 m) upstream. After flows of the summer rainy season, the channel was completely adjusted upsteam, but new bank erosion ensued downstream as the channel impinged at nearly right angles on the former bank below the cutoff. The cutoff meander was plugged at its downstream end by deposition of a broad levee from the new channel and isolated at its upstream end by downcutting. Seepage through the coarse sand of the former channel fed clear water to the oxbow long after it was isolated.

At the second reach, floods and low flows episodically made minor changes in width and depth (± 1-2 ft; 0.3-0.6 m) of the inner channel and raised or eroded the adjacent levees and flood plain. The largest flood of the decade (3750 cfs, 106 m³/s, recorded at Bernardo) overtopped the levees by at least 3 ft (1 m), scouring the flood plain into smaller arroyo-like channels in one area while building bars and levees in another area. The inner channel doubled in depth in local pools by scouring at least 10 ft (3 m) below its former level and broadened 10-13 ft (3-4 m) at the expense of its semi-stabilized, tamarisk-covered banks. Flows after the big flood initially expanded erosion in the inner channel upstream and downstream from the big scoured pools, but within two years the large scours had filled in and new inner banks had developed over the scour margins.

Some of the depositional features of post-flood recovery are found in prehistoric cut-and-fill cycles, suggesting similar decadal-style floods and adjustments in the past.