Airphotos of San Luis

1935

1958

1973

1990

View all four images in an animated movie here.
(Caution! File is approximately 400 Kilobytes.)

A time-based animation of aerial photographs for the Rio Puerco near San Luis, New Mexico
Melvin H. Podwysocki
Erik Barron
U.S. Geological Survey
Reston, Virginia

Along this reach of the river, the Rio Puerco is entrenched and cut into valley fill sediments consisting mainly of sands and silts with minor amounts of gravel (Yount, personal communication). Stream channel walls range between 40 - 60 feet high and form nearly vertical cliffs. Prior to about 100 years ago, the Rio Puerco flowed on the top of the valley fill. Since then, it has entrenched itself, like many streams of its size in the southwestern U.S. Water flows intermittently in the channel throughout the year. Flow is most common during the spring snow meltoff, where the river receives water not only from the local region, but from the Nacimiento Mountains farther to the north, as well as from storm related runoff throughout the rest of the year.

Observations made from the aerial photographs suggest that different parts of the river have behaved differently. As an example, the channel near San Luis (at the northeast (right) edge of the photo sequence) changes relatively little over the intervening years. The only significant change viewed on the photos from this area show an increasing amplitude of two meander loops centered about 1.25 km southeast of the right edge of the photo sequence. Conversely, meanders farther downstream to the southwest, dramatically change both their amplitudes and locations, beginning at Arroyo Balcon, the first prominent stream on the airphoto sequence flowing into the Puerco from the northwest. The animated photo sequence shows the meanders propagating downstream, as is typical of fluvial courses (Fiske, 1944), but are impeded or "pile up" at junctions with major side drainages originating from the northwest. Arroyo Balcon, an unnamed arroyo about 2.5 km southwest of Arroyo Balcon, and Arroyo Chavez, just over 3 km southwest of Arroyo Balcon, all appear to affect the meander loop pattern.

There may be several causes for the piling up of the meanders, but all require that a gradient change be present. In effect, as the river's gradient lessens, an obstruction to the free movement prevents the continual propagation of the meanders downstream. The obstruction to the continual propogation may be the presence of more resistant sediments or a resistant bedrock ledge in the main channel bottom, or an increase in sediment load dumped from tributary streams into the main channel.