GUARD FIRST THE BOTTOM LAND

R. V. Boyle, J. S. McCorkle

OFTEN the heart of a ranch is its flood-watered bottom land. These acres of alluvial plains may comprise only one one-hundredth of the area of the ranch, but if they are in good condition they can produce 2 to 20 times more forage than do the adjacent uplands.

The flood plains often have significant value as a nursery for calving or lambing, a hospital for thin or ailing stock, a fattening pasture, a winter-feed pasture, or a hay meadow. On them the carry-over of soil moisture after flooding may insure green forage for months after the upland range has dried up.

Where only light runoff is available, grasses in the flood-irrigated valleys may be the same species found on the uplands although more luxuriant. The Most significant increase in production, however, occurs when the volume of floodwater is great enough that the short grasses are replaced by taller, deeper-rooted species. Forage production then may be measured in tons as forage from the uplands is measured in hundredweights.

That is what happens when the range is in virgin state or in good condition. But often it is not: Depletion has been severe in many places. Upland vegetation has deteriorated. Much of the original grass cover has disappeared. The shrubs and grasses which follow are less able to resist erosion, and flash runoff increases.

And on the bottoms and flood plains misuse has been most concentrated : Cattle congregated on them; roads and trails appeared; the vegetation was thinned out; and channels were cut. When these areas were in virgin condition, heavy floods dug out pot holes, which usually filled with silt and debris and were revegetated without permanent damage. But as the vegetative cover was depleted, the holes grew in length and gullies were formed, made worse by the ruts and channels in the roads and trails. The bottoms were drained. Through the gullies, silt that had slowly accumulated in the valleys for centuries moved in damaging quantities to the stream channels and often spread over farm lands. Damage downstream from range deterioration is difficult to estimate, but is considerable. Silt fills river channels and reservoirs, and flash floods may damage irrigation systems and other property.

One can divide southwestern watersheds fairly readily into silt-producing areas and water-producing areas. Heavy water-yielding areas are generally at high elevations that have relatively good precipitation and vegetation. Their silt yield is relatively low because the vegetation on them resists erosion and rains are less intense.

Less favorable elevations are and or semiarid, subject to long dry periods and torrential downpours. Soils are often erodible, and flash floods that carry excessive loads of silt are likely. Both flood and silt are greatly increased by range depletion. There are, of course, both uplands and valleys that have not suffered severe damage and others that have recovered.

Where damage has occurred, the remedy has two parts. The first is to correct the misuse responsible for depletion of the vegetation. The second is to restore flood-plain conditions by diverting the water from the gullies and to use whichever structures are necessary to insure a reasonable distribution or spreading that will control and prevent further gully formation.

It is usually impracticable to divert floodwaters to the uplands. The conditions required for successful diversion and spreading of water on range will usually be found on formerly flooded areas be they, a fraction of an acre or several thousand acres.

Several factors complicate the reclaiming of range bottoms. Water law is one. Irrigation of cropland has reached a high state of development, and the water from most western watersheds is appropriated for that purpose. Water laws in Western States are based on the principle of prior appropriation whoever puts a water supply to beneficial use has a right to its continued use under State regulations.

Court decisions are somewhat vague on the point, but benefits to a rancher from natural spreading of floodwaters over grassland are seldom if ever contested. After floodwaters accumulate in a natural channel of sufficient size and permanence to be recognized by the courts as a watercourse, prior, appropriations take precedence. Many large gullies now draining the range were caused by relatively recent activity of man rather than by natural geologic processes, and some authorities do not consider them "watercourses" in the legal sense. The rights of range operators to reclaim flood plains by diverting the water back to its original state of spreading over the area affected have not been generally defined.

Few specific data are available to determine the effect of this or other types of watershed treatment on total stream flow. Water that would drain off in a few hours through a gully may meander across a vegetated spreading area for days before finally draining off downstream, but there is little information to show the total yield of water in each case.

Diversions may vary from protective work to prevent further deterioration of natural spreaders to complete reclamation where gullies have been established for years. Many of the logical water diversions for range land occur on interior drainage basins where water appropriations do not introduce serious problems. In other instances the rights to the use of water for spreading may be recognized; it is possible also that the use of the water for farm irrigation is so remote that no question will arise.

Spreading floodwater on range is a type of irrigation. Such use of water may be less efficient than irrigation of cropland where flow is regulated and measured, but there are factors (such as saving water in transit) that are in favor of spreading. Flash floods may run off rapidly, where there are gullies and channels to carry them, but dry sand washes can absorb an enormous amount of water and may even entirely dissipate sizable floods before they join a permanent stream.

Other factors are of a physical nature.

Spreading may be impracticable because gully erosion has progressed too far. The arroyo may be too big or may carry too much water to handle on the area in question. Each site must be analyzed on its own merits, and its physical characteristics and limitations and engineering needs considered.

The type of diversion is determined chiefly by the size of expected floods and the size and character of the spreading area. Supplementary structures to effect a satisfactory spreading of water will vary with slope and topography. Small dikes, brush or rock percolators, wire diversions, or longer dikes with "weeps" to pass water in small quantities, are the types usually employed. Water should be in controllable quantities and flows frequent enough to saturate the soil one or more times a year.

An excessive amount of alkali in the soil is detrimental. Heavy textures and relatively impermeable soils may be suitable for water spreading if the gradient is flat enough that water will remain on them for some time long enough to soak in thoroughly. Coarse, gravelly soil is usually not satisfactory.

Some diversions onto areas with not more than 2 feet of good soil are successful, but generally there should be 4 to 6 feet or more of good soil in water-spreading areas. This will provide soil enough for sufficient water storage in the principal feeding zone for grass roots.

The topography and slope must be such that water 'will spread naturally or with a limited amount of diking and supplemental structural work. The Suitability of an area for spreading water will usually decline rapidly with "'crease in slope; 5 percent is about the maximum slope for practical application; 1 percent or less is better.

The type of vegetation already present is important. If there is a fair growth of perennial vegetation to retard and spread the water, greater amounts of water can be handled safely than if the cover has deteriorated. Seeding may be required on denuded areas or where desirable grasses have disappeared. For this purpose the native grasses found on the natural spreading areas are probably the best suited, but good seed of native species is not always available. Where reseeding is necessary, species should be selected for the site, considering alkalinity of the soil and the ability of the grass to endure flooding, silting, and drought between floodings.

Where a heavy silt load is expected, it may be practical to install stream-bottom fencing and gully plantings upstream from the diversion so that the heaviest silt will be deposited in the channel before it reaches the diversion. Sound range management (including proper use of the vegetation on the watershed) is, however, the most effective means of controlling silt.

Before a diversion is installed, one should plan the disposition of all the water to be diverted. If water flows out at the lower end of the area and drops into a raw gully, there may be danger of a head cut working back across the treated area. Occasionally small levees may be used to keep water out of gullies and supplemental outlet structures may be needed.

Getting the proper use of forage on spreading areas is often difficult. In dry periods, when the spreading areas contain the only green grass, animals tend to concentrate there and may seriously damage the cover. It is seldom possible to achieve proper use of the spreading area and adjacent range without fencing so as to control the use of each. Properly handled, the forage from the areas affected by water diversions will satisfactorily supplement the total range feed.

The importance of regular and systematic inspection and maintenance cannot be stressed too much. Water spreading by means of diversions is designed to be more or less self-operating as a rule, but no system can be expected to continue to operate without maintenance. The cost of maintenance will be much less if the system is observed closely and repairs and adjustments are made promptly. Rodents, for example, may burrow through dikes and cause destruction of the whole system unless repairs are made.

Unrepaired, a minor detail may become a major operation. Silting may necessitate adjustments such as shifts in location of spreader dikes. The main diversion is the key to the whole system and may be destroyed or made useless by lack of proper maintenance.

THE AUTHORS--R. V. Boyle, regional chief of operations with the Soil Conservation Service in Albuquerque, N. Mex., is a native of Arizona. He began work in 1925 in the Department of Agriculture as a forest ranger and a specialist in range management. Mr. Boyle has been with the Soil Conservation Service since 1934.

J. S. McCorkle, regional chief of the Range Division, Soil Conservation Service, in Albuquerque, is a south-westerner. He was employed for several years in animal husbandry work following graduation from college. He has been engaged in range conservation work with the Soil Conservation Service since 1935.