The Natural Environment

Introduction: Who we are on the Yelm Prairie is part the result of what has happened along the Nisqually River and on the land bordering it over thousands of years The following selections will provide a picture of the natural forces at work shaping the environment that Native Americans and Euro-American settlers discovered.

The Nisqually River: The following description of the Nisqually River is taken from a final report issued following hearings before the Federal Energy Regulatory Commission the summers of 1982 and 1983. it provides one with a working image of the modern, harnessed, river. The report reads in part:

The Nisqually River, a glacier fed river with a drainage area of 754 square miles, is located in western Washington. It flows for more than eighty miles from the Nisqually Glacier on Mount Rainier to Puget Sound. One of the ten major rivers which empty into the Sound, the Nisqually is the southernmost, and the fifth largest in terms of flow.

From its source, 14,500 feet above sea level, the Nisqually flows southwesterly for about five miles while dropping almost 2,000 feet in elevation. The river then flows westerly for five miles, falling at a rate of 100 feet per mile. At this point, near the Nisqually Park Ranger Station, the river flows northwesterly for 26 miles falling at a rate of 40 feet per mile, until it reaches the Alder Dam (the first two Tacoma facilities) which is located at approximately river mile 44.2. Downstream at river mile 42.5 is the second Tacoma project facility, the La Grande diversion dam. The river here is diverted downstream about 1.7 miles to the La Grande powerhouse, where the diverted water reenters the river channel.

Downstream of the La Grande Dam at mile 40.8, the Nisqually River flows almost 41 miles before reaching Puget Sound. In this last leg, the river has an average drop of about 12 feet per mile. It is in this reach that Centralia’s diversion dam is located at rivermile 26.2. The dam, a log crib structure, is approximately 8.5 feet high and 206 feet long at its crest. A concrete intake structure, located on the dam’s west side, diverts water from the river into Centralia’s 9.2 mile long, unlined earthen canal. This canal ends at the Centralia powerhouse, which is located at about rivermile 12.7. Centralia and Tacoma, pursuant to interim flow orders, ensure the almost 13.5 mile channel between the Centralia dam and powerhouse is provided sufficient water to support the anadromous fishery. The part of the river is called the “diverted” stretch.

The United States Army owns a significant amount of land along the diverted stretch. Part of this area comprises Fort Lewis. This land stretches on the south bank from rivermile 19 to rivermile 2.3. This stretch thus commences 6.3 miles above the Yelm powerhouse. On the north bank, federal lands cover rivermile 17.6 to rivermile 14, and rivermile 12.3 to the upper boundary of the Nisqually Indian Reservation at rivermile 11. The reservation extends down the rivermile 5.4.

The juncture of the Nisqually River and Puget Sound forms a delta which is referred to as the Nisqually Tidal Flats or the Nisqually Mud Flats. At this point, a tidal exchange of fourteen feet is experienced between high-high tide and low-low tide. Id. In spring, the magnitude of this exchange increases to sixteen feet. Normal low tide is about three feet and normal high tide is approximately right to nine feet. At high tide, the depth of water covering the flats is sufficient to allow a boat with a four to five foot draft to traverse the flats. A river channel provides access at all tide levels other than, perhaps, low-low tide. Tidal influence affects the stretch of the river between rivermiles three and four. This same portion of the river has been described as resembling a lake at high tide. (From : Federal Energy Regulatory Commission, Docket No. E-6454-000. Hearings were held in Seattle, Washington, on August 2-4, 1982, and July 25-August 5, 1983. At these hearings the Nisqually Indian Tribe, the Washington Departments of Fisheries and Game, Commission Staff and the City of Centralia presented testimony and numerous exhibits.)


Repeated advances and retreats of great continental glaciers from Canada have sculpted the entire Puget Basin. Reaching a point nearly 20 miles south of Olympia, these periods of glaciation occurred throughout the Ice Age (Pleistocene era, 1.6 million to 10,000 years before present). The glacier that once covered Olympia may have been up to 1,500 feet deep!

The last major lobe of continental ice, the Vashon ice sheet, receded from the area that is now the lower Nisqually River Basin about 13,000 years ago. Enormous amounts of glacial debris were left behind, and these filled the Nisqually Valley from about the Mashel Prairie at the foot of the Cascade Mountains to the delta. Glacial erratics, rocks transported from as far north as Canada, can be found throughout the mid and lower Nisqually Valley.

For hundreds of years, huge sediment-choked rivers streaming out of the receding Vashon Glacier deposited immense outwash plains in southern Puget Sound. Ice dams broke and catastrophic floods, slurries of mud and sand, raced in torrents over the landscape. These outwash plains are responsible for the prairies that lie adjacent to the Nisqually Valley from its boundary near Kapowsin to the Bald Hills and in the valley below.

In a region typified by extensive forests, these prairies are striking. Open, grassy, meadow-like land, dotted with flowers – they are a signature feature of the mid to lower Nisqually Valley. If you were to dig a soil pit in some of these prairies, the profile would be very revealing. Roadcuts show the thin loamy soil that has developed on top of the coarse, cobbly glacial till left behind by a prehistoric earth mover – the Vashon Glacier. While this soil appears black and fertile, it is in reality not rich.

Though some cultivation occurs, much of this Nisqually prairie land is today utilized as pasture. Despite growing urbanization, the Yelm area is still known as Yelm Prairie. The best examples of these prairie lands are today found within the boundaries of the Fort Lewis Military Reservation.

Even where gravelly outwash areas are overlain with forest, soils are highly porous.

Consequently, groundwater is easily recharged, resulting in a water table that is typically only about forty to fifty feet deep in the prairie areas. The permeability of the subsurface enables groundwater to flow readily in its migration toward the river. In certain cases, this enhances the problem of failing septic systems, whose effluent can quickly reach the river and its tributaries.

Where depressions in the bottom lands of the valley are lower than the water table, windows in the aquifer occur. The resulting ponds and marshes are the wetlands which dot the Nisqually Valley. Such swamps and marshes are vital habitat for many plants and animals.


Unusual to the watersheds of Puget Sound are the presence of open prairie-like lands which in other Northwest watersheds would be forested. These prairies occur in the gravelly outwash plains of not only the Nisqually, but also the Deschutes and Chehalis Rivers. Early explorers and settlers to the area must have been surprised to break out of thick forests and into the patchwork quilt of the prairie lands.

Yelm Prairie pioneer James Longmire, who left Shawnee Prairie, Indiana, to travel the overland trail to Nisqually country in the mid 1800s, naturally gravitated to such familiar country. Here were flat and rolling plains of thick turf dotted with wildflowers such as camas and interspersed with groves of Douglas-fir, Oregon white oak and even ponderosa pine. This inviting appearance soon proved to be deceptive. Longmire and others soon discovered that unlike midwestern prairies which had deep and rich soils, Puget Sound “prairies” overlay coarse, gravelly glacial till. These early settlers to southern Puget Sound noted that the Indian people burned the prairies in order to ensure a good crop of camas, a flower in the lily family whose underground bulbs were baked and boiled. These deliberate bums also attracted game at night. In doing so, these stewards of the land destroyed invasive tree seedlings and maintained the open prairie.

Mature trees are another striking feature of the Nisqually prairies which dot the lower valley. Interspersed as either individuals or in groves, these trees are unique. Douglas fir, tall and clear of limb for one hundred feet or more in the cramped quarters of the lowland forest are here laden with branches which sweep the prairie grasses. The only oak indigenous to Washington, Oregon white oak, is common in this habitat. Within the long-standing military reserve of Fort Lewis are groves of rare native ponderosa pine. These 250-year old trees are very rare on the west side of the Cascade divide.

The oak prairies have always been host to a myriad of animal species. Black bear, bobcat, cougar, raccoon, Douglas squirrel, western gray squirrel and black-tailed deer were all common to Nisqually prairies. When the Hudson’s Bay Company arrived in the Nisqually Basin in the early 1800s, so too did domestic livestock. Unlike deer, sheep and cows concentrate in large numbers, often overgrazing the land. With its protective layer of turf removed, tree seeds could easily take root and the balance between grove and prairie shifted. The settlement of the Nisqually valley by pioneers and the subsequent elimination of wildfires and Indian bums further favored the encroachment of trees into the open prairie. With the introduction of exotic grasses and more damagingly, scotch broom, Nisqually prairies are today a fraction of their historical size.

Riparian Zone

The vegetated area bordering the shores of streams, rivers, lakes and ponds is called the riparian zone. While this zone comprises only a small part of the watershed, it is very rich in plant and animal species. It has a profound influence on the aquatic ecosystem and, in turn, is heavily influenced by an abundance of freshwater. This abundance of water favors shrubs and deciduous hardwood trees, though cedar and fir line the banks of the river as well. Cottonwood and maple trees are common. Well-watered, rapidly growing willows and alder are often shrub-like in this lush streamside habitat. Devil’s club, thimbleberry, ferns, rushes and reeds crowd the banks. On smaller creeks and streams the shade these plants provide moderate the water temperature, keeping it cool on hot summer days. (From: The Living River: A Guide to the Nisqually River Basin 1996) Used with permission.

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