During the economic decline of the 1930s, the American government employed young men to build and restore parks and wilderness areas throughout the country. This program was known as the Civilian Conservation Corps (CCC) and it was a huge success.
In southeastern Arizona, CCC crews worked in mountain canyons to control flash floods (sudden rushes of water carrying rocks and shrubs and cars down canyon). These crews built many stone retaining walls -- small dams, essentially -- to trap the water and prevent flooding.
The other day a friend and I walked up Zacatecas Canyon and passed a number of these retaining walls. They were beautifully constructed and well-placed to control the flow of water.
And they were complete failures. Because flash floods carry so much debris, it took only a few years for gullies behind the walls to quickly fill with dirt.
I'm sure these walls seemed like a good idea at the time. But the forces of nature have an inevitability about them. We can tinker with these forces and perhaps re-direct them for a short time. But cause and effect always wins out.