A highly eroded landscape, overrun with weeds: Almost all of the plants in this picture are invasive exotics, even the dead annuals in the foreground.
One thinks of water as a good thing in the desert. However the changes that have occurred in the Wash have not been painless. As can be seen in the photos below, conditions are very different in a dry wash than along a perpetually flowing stream. The vegetation that previously dominated the site was not well suited to the changing circumstances. As a result, when increasing stormflow led to floods, plants were carried away and the banks eroded. Rising water tables and newly exposed soil did not provide a hospitable environment to the seeds of the plants that remained, as it was no longer their preferred environment.
_A dry wash, nearby in Lake Mead NRA___The Las Vegas Wash
Meanwhile the increasing human population also brought new plants into the valley (some intentionally, and some accidentally), many of which were better suited to the wetter conditions than were the original species. These took root and began to compete with the native vegetation. Given the altered environment, in many cases it was no longer a contest.
Now invasive weeds make up most of the vegetation in portions of the Wash that have yet to receive aggressive management. Of these the most dominant is Saltcedar. It tends to form thickets that shade out and exclude other plants. It also concentrates salts in it's foliage which then build up in the soil underneath it, further discouraging competition. Once stands get thick enough, they burn easily, which is not a problem for Saltcedar as it is fire adapted and recovers quickly.
Ironically it was originally introduced to the desert southwest intentionally, as an ornamental and to combat soil erosion. However now it is out of control, and growing in densities that frequently make it a fire hazard. With shade, salt, and fire as allies, few things compete well with Saltcedar.
Inside a Saltcedar thicket ready to burn__ Saltcedar monoculture following fire
Continued: What is being done