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Figure 75. Endangered Moapa dace, 55 mm in total length, from the upper Moapa River, Nevada. Figure 76. White River springfish, 35 mm in total length, from Mormon Spring, Nevada. Various subspecies of this colorful species, most of which are considered threatened or endangered, are distributed in springs of the pluvial White River Valley, Nevada. Figure 74. Source pool of Mormon Spring, Nevada, 1981, a major habitat of White River springfish. The spring outflow leads to an extensive bulrush marshland. Desert springs are often difficult to see until one is immediately upon them. Soils surrounding them are typically too waterlogged or saline to support large, woody vegetation (Fig. 74), and highly tolerant sedges, grasses, and other hydrophytes (or halophytes) may have low growth-forms like that of surrounding desert vegetation. Little erosion occurs, and outflows commonly meander through undercut, sedge-choked solution channels. Bottoms are made up of organic materials and flocculent sediments formed through chemical action. A series of basins in eastern Nevada now holds a number of disjunct spring systems that once fed the pluvial White River (Fig. 1). This stream, which flowed south to enter the Colorado River, still supports a substantial fish fauna including chubs, speckled dace, spinedaces (one extinct), and a mountain-sucker clearly allied to species and populations of the lower Colorado River, along with a number of other kinds of more obscure relationships. One example of the last is the Moapa dace (Fig. 75), a small minnow restricted to thermal pools and their outflows of the upper Moapa River. This species is rarely found in water cooler than 300 C, and apparently prefers a temperature range of 31 0 to 340 C. It is classed as endangered because of its extremely limited distribution. The White River springfish (Fig. 76) also lives in thermal springs and outflows of this same system, and is equally as unique as the Moapa dace. This species is a near relative, or member, of a central Mexican family of livebearing fishes. It differs from Mexican species in retaining the primitive, egg-laying 38