The park is covered by Pleistocene till and Holocene alluvium on the west and north sides. Deposits of till are from terminal glacial moraines, and alluvium consists of bouldery fan, mountain basin gravel and flood plain sand and gravel deposits. South and east of Pearl Lake are exposures of Precambrian felsic gneiss and volcanic amphibolite.
Major native plant communities at Pearl Lake are:
- Lodgepole pine forest
- Northern subalpine forest
- Aspen forest
- Willow carr
- Wet meadow
Northern subalpine forest and lodgepole pine forest are the most common vegetation types at Pearl Lake. Northern subalpine forest, comprised of subalpine fir, Englemann’s spruce and Colorado blue spruce, exists more commonly on north-facing slopes. Herbaceous understory species in the lodgepole pine forest include broom huckleberry, geranium, strawberry, arnica and mountain brome.
The aspen forest community is characterized by a rich herbaceous understory of lupine, yarrow, geranium, meadow rue, wheat grass and Thurber fescue. Sagebrush shrublands occur on the northeast side of Pearl Lake. Big sagebrush dominates this community, though rabbitbrush and hoary sagebrush are other common shrubs.
Willow carr, marsh and wet meadow plant communities have become established around the reservoir, in Beaver Creek and in Lester Creek above and below the dam. Willow carr is a wetland shrub community dominated by a variety of willows including wolf, booth, geyer and shining willow.
Marshes are the most common wetland community at Pearl Lake and consist of sedges, rushes,and bulrushes growing in areas of permanent standing water. Small areas of wet meadow possess traits suggestive of periodic wet conditions, such as moist soil and hydrophytic vegetation. Common native species in the wet meadow include tufted hairgrass, elephantella, western bistort, shrubby cinquefoil and scouring rush.
Visitors may observe mule deer, as well as porcupine, red fox, pine squirrel, beaver or muskrat. Routt National Forest helps to preserve large tracts of contiguous habitat in the region. American marten, long-tailed weasel, tiger salamander, striped chorus frog and western terrestrial garter snake are also present.
Because dramatic late summer drawdowns do not occur at Pearl Lake, the reservoir exhibits characteristics similar to a natural lake with more constant shoreline conditions and water levels for migrating and resident wildlife. The restriction to wakeless boating on Pearl Lake contributes to the quiet and peaceful atmosphere at the park.
Over 200 species of migratory and resident birds are known in the park including northern harrier, osprey, great blue herons, western screech-owl, western bluebird, hairy and downy woodpeckers and red-winged blackbird. Greater sandhill cranes return to nest in the willow carrs and marshes in the spring. The park staff requests that visitors keep their distance when observing the cranes.
The cutthroat trout population at Pearl Lake is designated as Gold Medal. Brook trout, brown trout and grayling are occasionally reported.