A small population of coastal cutthroat trout resides in the stream, which in 2005 was the only major water body in Portland that met state standards for bacteria, temperature, and dissolved oxygen.

Historic Guild's Lake occupied part of the lower watershed through the 19th century, and in 1905 city officials held the Lewis and Clark Centennial Exposition there on an artificial island.

Soon thereafter, the stream receives an unnamed tributary on the right bank and turns southeast on private property along Northwest Cornell Road.

[11] Near Northwest Thurman Street, roughly 1 mile (1.6 km) from the mouth, the creek flows through a trash rack into an 84-inch (210 cm) diameter storm sewer.

[14] The Bureau of Environmental Services (BES) of the city of Portland monitored the flow of Balch Creek from June 1996 through September 2002 at a site, Node ABB857, where the stream leaves the surface and enters a storm sewer in Macleay Park.

[6] Measurements taken during an individual spring, from mid-May to mid-July 2002, showed the flow starting at about 2.5 cubic feet per second (0.071 m3/s) and dwindling to 0 by early June.

[6] Solidified lava from Grande Ronde members of the Columbia River Basalt Group underlies the Balch Creek watershed.

About 16 million years ago during the Middle Miocene, the Columbia River ran through a lowland south of its modern channel.

[16] Geologists have identified several basalt flows in the West Hills, where they underlie the steepest slopes of Forest Park and form the columned rocks visible in parts of Balch Creek Canyon.

By the 1830s, diseases carried by white explorers and traders reduced the native population by up to 90 percent in the lower Columbia basin.

Although large parts of the land remained undeveloped until the early 20th century, lumber mills, grain storage structures, railroads, and docks appeared along the waterfront.

[19] In 1905, the Lewis and Clark Centennial Exposition, held on an artificial island in Guild's Lake, helped spur growth in the area.

After the exposition ended, developers filled the lake and its surrounds with soil sluiced from parts of the Balch Creek watershed in the West Hills above the floodplain or dredged from the Willamette River.

After World War II, chemical and petroleum processing and storage, metals manufacturing, and other large industries expanded in the area.

[5] Nearby watersheds include those of other small streams flowing directly into the Willamette along the east flank of the West Hills.

[30] In 2002, aerial photographs showed that buildings, parking lots, shoreline structures, and other cleared areas covered most of the lower floodplain.

[32] Forest Park and other areas of the watershed have an understory of well-developed shrubs including ferns, Oregon-grape, vine maple, salal, red huckleberry,[33] Fendler's waterleaf, Indian plum, salmonberry, and stinging nettle.

[34] Among the prominent wildflowers are wild ginger, Hooker's fairy bells, vanilla leaf, evergreen violet, and trillium.

After people filled the wetlands and diverted the lower stretch of the creek into a pipe, fish could no longer migrate to and from the Willamette River.

[30] Sixty-two mammal species, including the northern flying squirrel, black-tailed deer, creeping vole, bobcat, coyote, Mazama pocket gopher, little brown bat, Roosevelt elk, and Pacific jumping mouse use Forest Park.

[38] blue grouse, great horned owl, hairy woodpecker, Bewick's wren, orange-crowned warbler, osprey, and hermit thrush are among the 112 species of birds that frequent the area.

[38] Amphibian species seen at the Audubon Society pond include rough-skinned newts, Pacific tree frogs, and salamanders.

Invasive plant species such as English ivy have made the habitat simpler and less supportive of native insects and the salamanders and other amphibians that feed on them.