The Las Vegas Wash riparian ecosystem: development, degradation, and active rehabilitationSeth A. Shanahan and Keiba K. Crear

Southern Nevada Water Authority Riparian landscape rehabilitation is often conducted in areas that have been negatively affected by anthropogenic disturbances or some other similarly resulting stochastic environmental event. These activities typically aim to assist the recovery of a degraded ecosystem back to some historic or pre-existing condition. In Las Vegas, Nevada, rehabilitation efforts are currently underway along an approximately 6-mile section of the Las Vegas Wash riparian ecosystem, however, they do not aim to recover a historic or pre-existing condition. Prior to modern settlement of Las Vegas, the Las Vegas Wash lacked the fundamentally important requirement of a riparian landscape, water. Over the last 50 years, the Las Vegas Wash developed into a riparian ecosystem because of water discharge from human activities within the Las Vegas Valley. Suitable hydrologic conditions were created for a variety of native riparian plants. Without a substantial native seed source, however, non-native invasive plants (i.e., salt cedar) came to dominate the floodplain. Since the 1950s, the huge increase in urban development of the valley has created periodic flood events and increased base flow that has led to the degradation of the developed riparian and wetland. Today rehabilitation efforts are focused on stabilizing the main channel, and revegetating banks and bottoms with native wetland and riparian plants. To help guide rehabilitation, historical botanic records within the watershed, along with records from regionally similar riparian ecosystems, have been assessed to guide the selection of plants for the restoration effort. Effective revegetation techniques have included using container stock, pole cuttings, and wattles. More than 40 acres have been revegetated since 1999 and approximately 176 more acres are projected over the next 10 years, making this one of the largest riparian rehabilitation projects in Southern Nevada.

Abstract

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Development Degradation Active Rehabilitation

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The Las Vegas Wash (LV Wash) is located southeast of metropolitan downtown Las Vegas, Nevada in the southeast portion of Clark County (Figure 1 and 2). The LV Wash, a small tributary to the Colorado River, drains the ~1,500 square mile Las Vegas Valley hydrographic basin into Lake Mead (Figures 3 and 4). The LV Wash is a northwest by southeast trending stream channel that developed along a narrow zone of structural disconformity during the Pliocene.

Urbanization of Las Vegas in the 1930s, 1940s, and 1950s led to increasing discharge of surface water flows to the LV Wash. Additional water in the channel created ripe conditions for wetlands to develop. By 1955, areas that were previously dominated by phreatophytes were transformed to include an extensive wetland vegetation component, consisting of Tules (Schoenoplectus acutus) and other wetland species. In the decades to follow, the developed wetland has proved to be important for both water quality improvements and wildlife habitat.

Prior to 1942, ground water provided the major source of water for the residents of Las Vegas Valley. After this time, Colorado River water was pumped into the valley to meet both municipal and industrial needs. As the population of Las Vegas increased, so did the demand for water from the Colorado River, which resulted in further discharge through the LV Wash (Figure 6). Discharge, however, was not solely composed of municipal waters; urban runoff, stormwater, and shallow ground water were also components of the flow. These four flow components continue to be important today.

By the early 1970s the expansive wetland environment that was formed by the increased flow started to degrade because of the further increase in flow resulting from population growth. Stormwater flows particularly affected the LV Wash by incising the stream channel and eroding the banks of the floodplain (see photo above). The lowering and draining of the shallow ground water table, as the channel became entrenched, significantly reduced the wetlands that had been created. By 1995, wetland vegetation had been reduced to the periphery of the flowing channel (Figure 7). In addition, riparian areas that had previously been dominated by Prosopis spp. were degraded by the invasion of a non-native species, salt cedar (Tamarix sp.).

An ecological analysis of the LV Wash prepared by Bradley and Niles (1972), documented many of the fish and wildlife species that had come to benefit. They found that at least 237 vertebrates (2 fish, 6 amphibians, 29 reptiles, 39 mammals, and 161 birds) occur in the vicinity of the LV Wash.

Since 1999, the Las Vegas community has been engaged in a comprehensive rehabilitation project along the LV Wash. Activities are coordinated through the Las Vegas Wash Coordination Committee, a 28-member group of Federal, State, and local agencies, citizen groups, stakeholders, and private businesses and organizations.

The first step towards rehabilitating the LV Wash is to protect the channel from further erosion. Small dam like structures (Figure 8) are built to dissipate flows and the impoundments that are created provide favorable hydrology for wetland and riparian plants. These areas are actively planted with a variety of native wetland and riparian taxa.

Several regional nurseries grow containerized plants for the rehabilitation effort and cottonwood (Populus fremontii) pole cuttings and willow (Salix exigua) wattles have been used with good success (Figure 9). Over 40 acres of riparian and wetland habitat have been rehabilitated and an additional 176 acres is expected in the future. Rehabilitated areas are already providing habitat for regionally important wildlife including the Yuma clapper rail, peregrine falcon, willow flycatcher, Townsend�s big-eared bat, western red bat, and western yellow bat.

Entrenched channel of the LV Wash

Figure 7: Wetland vegetation along the periphery of the channel

Salt cedar (Tamarix ramosissima)

Yerba mansa (Anemopsis californica)

Saltgrass (Distichlis spicata)

Alkali bulrush (Schoenoplectus maritimus)

Coyote willow (Salix exigua)

Figure 8: Dam like structure and active rehabilitation areas

Figure 9: Cottonwood (Populus fremontii) pole plantings: 1 month (left) and 1 year (right) after planting

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Historical accounts of the LV Wash indicate that it was mostly an ephemeral desert channel prior to modern settlement of Las Vegas Valley (ca 1905). It did, however, support areas of dense phreatophytic vegetation comprised of the following genera, Prosopis, Pluchea, Sporobolus, and Distichlis. Malmberg (1965) delineated the aerial extent of these phreatophyte communities for the lower LV Wash. Figure 5 depicts the pre-development (i.e., 1905) extent of phreatophytes within the Clark County Wetlands Park boundary.

Figure 5: Extent of phreatophytes within the Clark County Wetlands Park in 1905

Clark County Wetlands Park

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Distichlis-Sporobolus

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Figure 6: Population growth and LV Wash flows

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Tule (Schoenoplectus acutus)Honey mesquite (Prosopis glandulosa var. torreyana) Hand drawing of LV Wash by John Steele (1855)

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Clark County Wetlands Park

Las Vegas Wash and Tributaries

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References: Bradley, W.G. and W.E. Niles. 1972. Study of the impact on the ecology of Las Vegas Wash under alternative actions in water quality management. Final report to the Las Vegas Valley Water District. Malmberg, G.T. 1965. Available water supply of the Las Vegas ground-water basin. U.S. Geological Survey Water Supply Paper 1780.