Water Management

Irrigation

Most irrigation diversions in Owens Valley began before 1890; by 1904 it was estimated that over 75% of the annual runoff in the valley was diverted for irrigation.^[11] These diversions reduced the total discharge of Owens River and altered its natural regime: maximum and minimum flows were both reduced downstream by large summer irrigation diversions, but winter flow was almost doubled by agricultural drainage.^[12] Flood frequency and magnitude doubtless diminished as well.

These changes may have affected regeneration of willows and cottonwoods, whose reproduction is tied to the natural runoff regime. Reduction of maximum and minimum flows could have decreased the area of freshly exposed alluvium available for colonization during the growing season or shifted the zone of seedling establishment downward in the channel, where young plants would be more susceptible to damage by the increased winter discharge. Consumptive use of water by agriculture would also have decreased the total water supply to riparian vegetation downstream. The effects of irrigation have since been obscured, however, by changes associated with more recent water management practices.

The Los Angeles Aqueduct System

Operation of the Los Angeles Aqueduct system began with completion of the aqueduct in 1913. The aqueduct had little effect on Owens River above the intake until 1929, when Tinemaha Dam began to regulate flow to the aqueduct at the Poverty Hills. The rest of the upper river remained unregulated until 1941, when Long Valley Dam was completed at the head of Owens River Gorge. Pleasant Valley Dam was added in 1957 to smooth out fluctuations caused by powerplants below Long Valley Dam.

Natural river discharge has been increased above the intake by interbasin transfers and groundwater pumping. The Mono Basin extension of the aqueduct system began diverting water to Owens River at Long Valley in 1941, and diversions increased following construction of the Second Los Angeles Aqueduct (located outside the study area) in 1970. These increases can be seen in the record of annual river discharge at Pleasant Valley (fig. 5).

Figure 5.
Annual discharge of Owens River at
Pleasant Valley and Keeler Bridge.
Source: unpublished streamflow records, LADWP.

During a prolonged dry period in the twenties and thirties, Los Angeles drilled approximately 170 wells in Owens Valley to increase supply to the aqueduct (Los Angeles Department of Water and Power 1966). Many of these wells were located north of the intake and discharged water into the river channel rather than the aqueduct. Pumping occurred from 1919 through 1935, then halted until the dry years 1960–62 (Los Angeles Department of Water and Power 1979). More wells were drilled after completion of the Second Aqueduct, and pumping now occurs every year.

South of the aqueduct intake, Owens River and most other streams have been diverted by the aqueduct since 1913, though some water continued to flow down the lower channel past the intake in most years until Tinemaha Dam provided a means of storing high flows. Since then water has been shunted downstream on just a few occasions, mostly when runoff has exceeded system capacity: in 1936–39 (before completion of Long Valley Dam), then again in 1967, 1969, and 1975.^[13] The 1938 and 1969 flows were exceptionally large, as can be seen from the record at Keeler Bridge east of Lone Pine (fig. 5).

Groundwater seepage has maintained some surface flow in the lower channel south of its intersection with the 1872 earthquake fault. This discharge has been quite small, often dropping to almost nothing from July through September, but relatively constant from year to year when no water is released past the intake.

Vegetation change associated with operation of the aqueduct system should be greatest in the section of channel between the aqueduct intake and the river-fault junction, which has remained dry since 1929 except for scattered seeps and irregular flood discharges. Field data (tables 1

[11] U.S. Reclamation Service. 1904. Report on the Owens Valley, California. Unpublished report by J.C. Clausen. On file at the Los Angeles Public Library, Water and Power Branch.

[12] Unpublished streamflow records, LADWP.

[13] Unpublished streamflow records, LADWP.

and 2) support this conclusion: diversion of surface flow appears to have eliminated most of the native riparian woodland from this area, except for the species least closely associated with riparian zones. Riparian taxa persist mainly in the channel bottom just north of the fault, where groundwater is most readily available. South of the fault, the small surface flow provided by spring discharge has evidently been sufficient to maintain much of the original riparian cover.

Establishment of salt cedar and Russian olive in the woodland is also best explained as a consequence of aqueduct system operation. Although salt cedar was perhaps naturalized near the river by 1944, local residents say that it became common only after flooding of the lower river in 1967 and 1969. The shrub spread rapidly into flooded areas, becoming especially dense in the dry section below the intake. Salt cedar is similar to the native willows and cottonwoods in being adapted for establishment after such fluvial disturbances, but here as elsewhere in the Southwest it has been better able than the native flora to colonize a habitat created by alteration of the natural runoff regime. Its success may be partly attributable to such inherent ecological advantages as more prolonged annual seed production and lower moisture requirement than the native riparian species (Horton 1977), Water was released down the old channel during the entire summer of both 1967 and 1969, perhaps delaying most seedling establishment until only salt cedar was still dispersing seed. On the other hand, willow and cottonwood seedlings may have been present initially but failed to survive without a permanent water supply.

When Russian olive reached the river is not certain, but the small size of all individuals observed during field work suggests that it too may have become established since 1969. It has not become as widespread as salt cedar, perhaps because its olive-like fruits are not wind dispersed.

Above the aqueduct intake, operation of the aqueduct system has changed the natural regime of Owens River (fig. 6): instead of cresting early in summer and falling rapidly to a late-summer minimum, the river now remains high from spring through fall, and maximum monthly discharge often occurs at the time of former low water. Flood magnitide and frequency have decreased, because flood discharge can be diverted or stored upstream. Similar changes in natural river regime have been found to hinder reproduction of riparian species elsewhere by eliminating the substrate necessary for seedling establishment (Johnson etal . 1976), and may partly account for the scarcity of young cottonwood and tree willow seedlings along the upper river channel. (The single exception, at site 8, is difficult to explain since other, apparently similar sites are devoid of young trees, but it does show the potential for seed reproduction where favorable conditions exist.) Seed reproduction appears more active in the channel itself below the earthquake fault, the only area in which river discharge still rises and falls more or less naturally.

Since 1970, tree regeneration along the upper river may also have been reduced by erosion associated with increased import for the Second Aqueduct. Bank slumping has in many areas produced vertical, unstable channel sides on which seedlings are unable to take root.

Figure 6.
Mean monthly discharge of Owens River at Pleasant Valley.
Source: unpublished streamflow records, LADWP.