|
A two year drought in 1933-34 reduced flows to zero for 4 consecutive months. In 1942, two successive flood events caused Elephant Butte to spill. (Peak discharge was ~57,000 cfs and flows in excess of 20,000 cfs persisted over 14 days in August and September. Anecdotal evidence from local residents credit the 1942 flood event with dramatically increasing the occurrence of tamarisk. Channel widths narrowed from 30 meters to 10 m during this period. An extended drought (1951-58) caused zero flows for 10 consecutive months (52-53) and 22 of 24 months, total. Elephant Butte storage declined drastically during this period. By 1955, the reservoir's effective irrigation supply was zero.
The effects of such a long drought on the regional ecosystem is relatively unknown. A reasonable suspicion is that it doomed the native riparian vegetation by adding declining water tables to competition from tamarisk (which is better able to "seek" water by extending roots in synch with falling water tables). TDS and salinity reached extreme levels during the long drought (7400 TDS/ 2400 Na were typical readings at Ft. Quitman during 1953-54). TDS/salinity recovered after 1958, but remained well above pre-drought levels, until the wet period of the 1980's.
Between the record low discharge of <12 kaf in 1954 and 1975, there was zero flow at Ft. Quitman 30% of the time (IBWC, 1978). This is attributable to a combination of full irrigation utilization, drought and increased riparian evapotranspiration during the peak season of plant activity, factors to which the reach is still subject.
1976 to present- This period may be characterized as the "golden age" of post-Elephant Butte river conditions. Ft. Quitman streamflows in the 1969-98 period average >165 kaf, compared with <100 kaf, 1943-69. TDS/salinity has been reduced to <2000/<400 (as a general range @ Ft. Quitman). Brief periods of zero flows occurred in May and June of the drought) years of 1977-78, but and otherwise were generally >25 cfs. (One curious water management aberration occurred in 1987 when 10 consecutive days of zero discharge were flanked by long periods > than 400 cfs). Of the 13 most recent years (1986-98), 7 featured peak discharges in excess of 1000 cfs. Two high flow episodes occurred during the cottonwood seeding period (May-June), one in July and four in October. Four were attributable almost entirely to releases of "surplus water" from Elephant Butte. Extirpation of cottonwood in the segment became virtually complete.
Conclusions- The ten years 1933-42 witnessed rapid environmental degradation in the Forgotten River segment, as flow regulation, drought and spread of tamarisk altered the ecosystem. An extended drought in the 1950's lowered water tables and accumulated salinity in the soils. Streamflow regimes at Ft. Quitman have been haphazard and subject to regulation for irrigation.
Since 1915, the average river channel has narrowed from ~100m to ~10 m. No attempts have been made, historically, to manage flows for ecosystem purposes. The segment remains vulnerable to relatively long, though infrequent, periods of zero flow.
Under the present regime, even wet year peaks arrive at times that are contrary to a natural shaped hydrograph, and thus tend to maintain the region's poor ecological status quo.
During the region's frequent water-scarce periods, minimum flows often contained excessive amounts of dissolved solids, salinity. However, given the overall availability of water in the water years 1986, 1987, 1995 and 1997, it is likely that, if restoration had been a management objective, flow targets might have been achieved.
Preliminary Streamflow Targets
The overall picture for this segment is loss of hydrologic and biotic equilibrium. The river channel does not maintain itself under existing conditions. Such floodplain as exists is covered with a monoculture forest of Salt Cedar (tamarix spp). Wildlife habitat and economic uses in the segment have been seriously diminished. Evapotranspiration from the tamarisk thickets and conveyance losses reduce outflows at Candelaria by about 50% from inflows at Ft. Quitman (IBWC, 1978).
Active vegetative and streamflow management will be required, if the Forgotten River segment is to be rehabilitated. Hydrologist/restorationist Jim O'Brien, of FLO Engineering, has suggested an adaptive regime of streamflows to the segment, based on experience in the sections of Rio Grande above Elephant Butte.
O'Brien's estimates are used here as our target in assessing the prospects for making adequate water available for the proposed restoration project.
This goal may be characterized as an attempt to "mimic the natural hydrograph" as nearly as possible, by:
Providing discharge peaks of 3,000 to 5,000 cfs, for 3 to 5 days duration (with gradual up-and down-ramping of flows), at a frequency of Five to 6 times per decade (with May or June timing preferred). We also assume that minimum flows of >0 are equally desirable.
|
