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t t concentrations. They suggested that phosphorus may be a limiting nutrient in the post-dam river below Glen Canyon Dam. The water guality of unregulated tributaries in the Grand Canyon reach fell into several classes (Kubly and Cole 1979). Most tributaries had dilute dolomitic (calcium-magnesium bicarbonate) waters, including Vasey's Paradise, Redwall Moss Spring, and Clear, Bright Angel, Shinumo, Stone, Tapeats and Deer creeks. The aquifers of these tributaries lay in Devonian and Mississippian limestones. A few springs emerged below high water line at river kilometer (rk) 48, 51.5, and 293. Tributaries with impure dolomitic waters included Havasu Creek, Lava Springs, Three Springs Canyon, Spencer Creek and Emory Falls, whose aquifers lay in the Muav Limestone. Sodium bicarbonate waters typified Diamond Creek. Sulfate waters characterized the Paria River and Kanab Creek, which derive sulfate from Jurassic and Triassic gypsiferous materials. Royal Arch Creek also fell into this category. Saline sodium chloride waters are contributed to the system by Pumpkin Spring and Blue Spring. The latter spring in the Little Colorado River "serves as a discharge point for abou~ two-thirds of the groundwater in the aquifer of the 28,000 mi Black Mesa hydrologic basin (p 570)." Despite this ionic diversity, Maddux et al. (1988) concluded that tributaries had little effect on river water chemistry. Havasu Canyon in the Grand Canyon is a beautiful example of the influence of water quality on aquatic and terrestrial habitat. The turquoise water of Havasu Creek is spring-fed, emerging from Pennsylvanian Supai group parent rock 1.5 km upstream from the Havasupai Indian village of Supai. The creek water is rich in CaCO~ which is precipitated as calcite on any obstruction to streamflow (leaves, twigs, logs, and rocks), and creates innumerable travertine dams throughout the canyon. Calcite is also deposited as an evaporite on canyon walls and vegetation by the spray emanating from its many dramatic waterfalls. Travertine dams create deep pools in which speckled dace and native catastomids breed, as well as slow-moving reaches in which riparian soil and profuse vegetation develop. Travertine dams are scoured out by flash-flooding, disturbance which tends to coarsen and homogenize channel characteristics (Black 1955). In contrast with Havasu Canyon, ephemeral tributaries (e.g., Beaver Canyon) are boulder-strewn and largely devoid of riparian soils or vegetation. t . . . . . Water Pollution . Salinity Of the many water-related problems confronting Colorado River water users, salinity has been the most enduring {U.S. Department . 24 .