Castle Lake and the surrounding area contain a wide variety of animals and plants including trout, bears, deer, otters, frogs, and osprey.

[5] The area is also well-known[6] [7] for early summer displays of flowers,[6] including red Columbine (Aquilegia truncata), fawn lily (Erythronium sp.

[19] At this point, the well-known Pacific Crest National Scenic Trail is one-half mile (1 km) away, just over the Trinity Divide.

Bradley,[26] a locally prominent mountain with views of Mount Shasta, overlooking Dunsmuir, California and the canyon of the Upper Sacramento River.

[29] Eventually the hollow took the shape of a large bowl in the side of the mountain, with the headwall being weathered by constant freezing and thawing, and eroded by plucking.

[28] The bowl shape extended to its northeast edge, where there was a terminal moraine of smaller rocks and debris which had been deposited by the glacier.

The Okwanuchu were speakers of the older Hokan-speaking family of languages, with archaeological sites associated with their range dating back more than 5000 years.

[35] The first non-Native Americans to pass through the area of Castle Lake were hunters and trappers of the Hudson's Bay Company, who began to come down the Siskiyou Trail from present-day Washington into Mexican-controlled California, in the late 1820s and early 1830s, in search of beaver and other pelt-bearing animals.

[36] At about this same time, Ewing Young led the first group of Americans up the Siskiyou Trail, passing near Castle Lake.

[38] Following the annexation of California by the United States in 1848 as a result of the Mexican–American War, Castle Lake came under the control of the U.S. Federal Government as public land.

The brook char was also originally stocked but now has become self-sustaining, and reproduces naturally in springs found on the eastern side of the lake.

[5] Other birds in the area include bald eagle (Haliaectus leucoephalus), golden eagle (Aquila chrysaetos), peregrine falcon (Falco peregrinus), wood duck (Aix sponsa), belted kingfisher (Megaceryle alcyon), Steller's jay (Cyanocitta stellere), and sooty grouse (Dendragapus fuliginosus).

[46][47] Reptiles and amphibians around the lake, including the rough-skinned newt (Taricha granulosa), the Cascades frog (Rana cascadae) and the garter snake (Thamnophis sirtalis), feed as well on insects and young fish (among other prey).

At and in the area below the lake, mixed forests of ponderosa pine (Pinus ponderosa), red fir (Abies Magnifica), white fir (Abies Concolor), lodgepole pine (Pinus Contorda), incense cedar (Libocedrus decurrens), and alder can be found.

This bacteria converts atmospheric nitrogen into soil-soluble nitrates which can be utilised by the alder, and favorably enhances the soil fertility generally.

[4] At higher elevations above the lake (especially in rocky or exposed locations), the pine and fir tree forests are thinner, and the trees grow in a more stunted fashion; instead, shrubs and bushes, such as green manzanita (Arctostaphylos patula), dwarf mountain manzanita (Arctostaphylos nevadensis), and tan oak (Lithocarpus densiflorus) find areas and niches to grow.

The best-known plant life in the area, however, are the lakeside and nearby meadow displays of alpine and sub-alpine flowers in the early summer.