RANGE WATERING PLACES IN THE SOUTHWEST 31 
the average period of usefulness of storage basins, before mud re- 
moval was needed, was nearly one-third less. In one instance, 
where the whole catchment area above a 5-year-old tank was badly 
overgrazed for several months, the volume of silt washed into the 
basin during the following year approximately equaled the deposits 
during the five preceding years. 
More mud has been deposited in many reservoirs during recent 
years, largely as a result of serious depletion of the vegetative car- 
pet on the watersheds by overgrazing and drought. Thornber (22), 
Wooton (29), and others have pointed out the increased erosion fol- 
lowing overgrazing. Investigations at the Great Basin Experiment 
Station also show a very clear relationship between overgrazing, de- 
pleted range, floods, and erosion (20) (&). Unusual floods, fires, 
cultivation, roads and trails, timber cutting, skidways, rodents, and 
similar causes, however, have all played their parts. Erosion dam- 
age to irrigated fields and crops and in reduced range production is 
an even more serious matter than the damage to reservoirs, but this 
discussion will be confined to ways and means of meeting the 
problem as it affects reservoirs. 
Pronounced dry periods of several weeks’ duration occur in the 
spring and fall over a large part of the Southwest. The top layer 
of soil becomes dried out and consequently dusty in exposed places 
or where the vegetation is thin. If the dry season ends with sudden 
violent downpours, even of short duration, the first storm waters 
which flow down many streams carry heavy loads of sediment and, 
in their first rush, sweep along much débris and litter as well, 
(Pl. XV, fig. 1.) On the other hand, slowly melting snows and 
gentle rains on heavily vegetated slopes cause a run-off that carries 
relatively little silt. 
When a storage basin has filled with mud, the question arises 
whether it is better to remove the mud or build a new reservoir. 
The possibility of finding another satisfactory dam site, the com- 
parative rates of silting, the quantity of mud to be removed, and the 
relative costs are the points that will usually decide the question. 
If the original dam site was wisely chosen, the location may not be 
easily duplicated. This may justify removing the mud, even at some 
additional cost, rather than building a new dam. Old storage basins 
usually have thoroughly puddled bottoms and are more nearly water- 
tight after the excess mud is removed than are new ones. If the 
excavation is carried below the former bottom, however, much of 
this advantage is likely to be lost. If a satisfactory new site can be 
found downstream the old dam will be of decided value for several 
years as a settling basin for catching silt, thus adding to the effective 
life of the new watering place. 
Cleaning an old reservoir may require the moving of a great deal 
more material than is necessary in building a new dam. The top 
layer of an exposed mass of silt bakes and hardens, but deeper down 
it usually remains saturated and sticky for months after the surface 
water has disappeared and is difficult to remove. (Pl. XV, fig. 2.) 
As a rule, the unit cost of moving this mud will be greater than 
moving material for a new dam. 
