The glacial flour settled to the bottom of the lake each year, forming a deep layer of pale material called glaciolacustrine silt. The river flowed north out of Lake Penticton into the Thompson and Fraser systems until the plug at McIntyre Bluff melted so that the Okanagan could flow once again into the Columbia. The shaping of the Okanagan Valley was complete—the glaciers had rounded off the plateau hills and cut the valley broad and deep. Like all fiord lakes, Okanagan Lake is very deep—about 232 metres at the deepest point. Meltwater streams filled other parts of the valley with huge amounts of sand and gravel carried down from the scoured plateaus. The glaciolacustrine silts form conspicuous white cliffs along the shores of Okanagan and Skaha Lakes but are noticeably absent south of McIntyre Bluff, the site of the plug that formed Lake Penticton.
Ecosystems
To make sense of the tremendous natural diversity of the Okanagan, it helps to divide the region into ecosystems. At the simplest level, there are four of these: the narrow ribbons of dry grasslands on the benchlands on either side of the valley bottom; the coniferous forests that stretch to the mountaintops; the islands of alpine tundra that cap the highest peaks; and the lakes, rivers, marshes, and other water-rich habitats.
The desert grasslands really characterize the Okanagan and set it apart from other lake-filled valleys in Canada. At first glance, these grasslands might appear to be rather monotonous and less diverse than the forests above, especially on a hot August afternoon when the dry soil crunches underfoot and the songbirds are silent. A closer inspection, however, quickly reveals a beauty and richness equal to any rain forest. The bunchgrasses here differ from grasses of the Canadian prairies in several ways, but primarily in that they actively grow only in spring, when soil moisture is adequate. By July the hillsides are parched, and the grasses, their nutrients stored in a large mass of fibrous roots, turn gold. The grasses wait out the summer drought and winter chill then green up again in spring. Many prairie grasses, adapted to moister summers, do not form distinct bunches but instead grow more evenly across the ground almost like a turf.
If you look at the grass itself, you will see that there are several kinds of bunchgrass common to the Okanagan, each indicating subtle differences in rainfall, temperature, and soil structure. Many valley-bottom grasslands in the south Okanagan are dominated by red three-awn grass, with its thick clumps of fine leaf blades and a three-parted seed head that looks exactly like the centre of a Mercedes-Benz logo. At slightly higher elevations, the southern grasslands are dominated by the tall tufts of bluebunch wheatgrass, with its seeds arranged on vertical stems like tiny beads on a string. This is the archetypal bunchgrass that fed the cattle industry, which in turn shaped the history of the West. Drier locations are often covered by needle-and-thread grass, whose seeds are essentially miniature spears that easily attach themselves to passing cattle and humans. Healthy upper-elevation grasslands in the southern valley, such as those on Anarchist Mountain east of Osoyoos and in the rolling hills around Vernon in the northern valley, are dominated by fescues, bunchgrasses with more feathery seed heads.
But perhaps the commonest grass of all in the Okanagan is a newcomer: cheatgrass. A native of southern Europe, its barbed seeds are shorter and more insidious than those of the needle-and-thread, filling socks and shoes with a mass of painful points. Cheatgrass is an annual plant, its seeds germinating in fall then flowering the following spring. The small plants don’t form bunches; their predilection for overgrazed sites has allowed cheatgrass to blanket entire landscapes across the Intermountain West.
Are these grasslands really a pocket desert, as the area around Osoyoos Lake is often called? The combination of sandy soils, cacti, rattlesnakes, and scorpions certainly fits the public perception of a desert. The annual rainfall is about the same as that of southern Arizona, and no one would dispute that the area around Tucson, with its saguaro cacti, is a desert. But many ecologists say that the presence of perennial grasses in the Okanagan Valley— the bunchgrasses mentioned above—indicates that these grasslands are a steppe. More particularly, they would call it a shrub steppe, because the grasses often grow in association with woody plants such as antelope brush and sagebrush.
Like the different grasses, each of these shrubs adds its own character to the ecosystem. Antelope brush is found in the southern half of the Okanagan Valley; its dark, gangly limbs are often associated with red three-awn, and together the two form one of the most endangered plant communities in the country. The sandy soils are dotted with clumps of brittle prickly pear cacti and serenaded by the songs of the lark sparrow. The present endangerment of this community is due at least in part to its increasing value as prime grape-growing land.
Sagebrush covers the dry hills of the southern valley as well, primarily on the west side. It seems to be found on more loamy soils than antelope brush and is often associated with bluebunch wheatgrass. Several species of birds and other animals are closely tied to sagebrush to the point where they are rarely found more than a stone’s throw from the aromatic grey shrubs. Sage thrashers nest throughout the dry western valleys of North America, north to the White Lake Basin just west of Okanagan Falls. Another songbird with a close attachment to sagebrush is the Brewer’s sparrow, which reaches the northern limit of its range just west of Penticton.
Throughout the grasslands there are scattered ponderosa pines—big, veteran trees with glowing brick-red bark and long green needles. As you climb off the benches and onto the surrounding hills, these trees become more common and quickly coalesce into park-like woodlands. This change from grassland to forest marks the lower tree line. Ponderosa pines are the tree species in this region most tolerant to drought and high summer temperatures. They are the classic conifer of the dry valleys and mountains of the West, evoking images of cowboys riding through sunlit glades. Like sagebrush, ponderosa pine woodlands host a suite of species found in no other habitat. The pygmy nuthatch is perhaps the bird most tightly bound to ponderosa pines; this small bird roams the forests in flocks that comb the furrowed bark and large cones for insects and other food items. A more enigmatic ponderosa pine specialist is the white-headed woodpecker, a strikingly patterned bird that specializes in eating pine seeds through the winter; in Canada this woodpecker is found only in the south Okanagan Valley.
Ponderosa pine forests have changed dramatically in the last century. Indigenous people burned these forests on a rotating schedule for thousands of years, setting fires on cool autumn days to clear out small trees and shrubs that encroached on the grassland understory. The large pines have a very thick bark, allowing them to survive the ground fires and maintain the open, park-like structure—large, scattered trees with a grassy under-story— so characteristic of these dry forests. When the West was settled by Europeans in the late 180 0s and early 19 0 0s, this method of forest management was abruptly stopped, replaced by logging of the large trees and active suppression of fires. Throughout much of the West—and the Okanagan is no exception—this has resulted in dense stands of young ponderosa pines, each fighting for scarce water resources and shading the light-starved plants on the forest floor.
As you climb higher on the hillsides, average temperatures drop and average precipitation increases, and with these differences comes a similar change in tree species. Douglas-fir gradually replaces ponderosa pine; then it, in turn, is replaced by western larch, Engelmann spruce, and subalpine fir. The plateaus on either side of the valley are covered largely by dense growths of lodgepole pine. Ecologists call lodgepole pine a seral species—a tree that pioneers open ground after a fire or logging, then gives way after a century or two to spruce and fir. In recent decades much of the lodgepole pine in British Columbia has been decimated by mountain pine beetles, and as a result, the forest industry has targeted it with massive clearcuts.
A few rounded peaks rise above the upper tree line in the Okanagan Valley. The forest opens up to green meadows with scattered whitebark pine; higher still, the trees are reduced to wind-pruned clumps of twisted branches, and flowers bloom from cushions tucked out of the wind behind lichen-covered rocks. This is alpine tundra, where summers are too short and too cool to allow tree germination.
In a valley with little rainfall, watery