OCR Text |
Show The Escalante River supports the most extensive native riparian community in the recreation area. This 50- mile river reach is not controlled by a dam, which allows periodic floods to control the potential invasion of tamarisk. The riparian vegetation along this river is therefore in a " near- natural" state: willow and cottonwood dominate, and tamarisk build- up occurs mainly along slackwater reaches below Coyote Gulch where the lake enters Escalante Canyon. The San Juan and Dirty Devil Rivers also support significant riparian communities in the recreation area. The major influences on flow of these three rivers originate outside the recreation area and include consumptive withdrawals for agricultural and municipal purposes. Potential water impoundment and mineral development sites surround the recreation area. Information pertaining to the effects of such outside influences on riparian ecosystems in the NRA is deficient. Canyon tributaries of the lake contain the greatest number of riparian zones in the park, and they are probably the most manageable from the standpoint of NPS programs. Many of these canyons are wholly within the park boundary; others have portions of their watershed outside the park. Surface flow in the canyons is intermittent, although subsurface flow may still occur during dry periods. Larger watercourses such as Halls Creek support perennial flow most years and can support small populations of native fish in the perch, minnow, and sucker families. Water must be available to plants during most of the year for riparian vegetation to develop, making water a basic resource issue in managing these specialized communities. Flow originates from seasonal surface runoff and from ground- water discharge into the canyons through springs and seeps. The water that sustains riparian biotic communities can therefore be affected by activities such as mineral development, road construction, or grazing. Mineral development may deplete aquifers or cause contamination. Road construction contributes to channel siltation and is a source of pollution from accidental fuel spills. The impact of domestic and feral livestock is presently the most serious adverse influence on canyon and spring riparian zones. Animals confined in the narrow canyons are drawn to the water and nearby forage, resulting in extensive trailing on slopes and fouling of the water. The effects of trailing are especially long- lasting because soil is destabilized and erosion cycles are often started that are very difficult to control. The effect on water resources is exerted through channel silting, increased runoff rates, and lowered water retention times. These effects reduce flow over the long term and are detrimental to riparian ecosystems. If placed in a riparian area to enhance flow, water developments for livestock could affect the water source by altering the flow regime. Additional information on sedimentation rates as related to disturbance by livestock would be necessary to document and mitigate these effects. The invasion of tamarisk along rivers is well advanced in the NRA. The high transpiration rate of this plant has been shown to reduce 40 |