Gardening: Ecological diversity marks I-90 drive

I traveled to the coast for Thanksgiving – a drive I’ve made several hundred times over the years but never get tired of. Why? Because of the ecological diversity I drive through across the Columbia Basin and through the mountains. Here are some of my observations as I traveled home.

Starting in the Puget Sound region, lush forests and prairies come right down to the highways. Moss even grows in shady areas on road shoulders. The forests become increasingly dense as you wind up Snoqualmie Pass. With the increasing elevation, fir trees yield to alpine fir and spruce. If it’s raining, it will rain harder on the west side of the pass as the rising clouds dump their loads to get over the mountains. Fortunately, Friday night’s snowstorm had melted into the shadows, and the road was clear.

The changes become even more obvious coming down the east side of the pass. The dense alpine forests quickly thin out and are replaced by ponderosa pine and cotton woods as you reach Cle Elm. Between Cle Elm and Ellensburg, the conifers retreat into the stream valleys surrounded by grassy prairies. By the time you reach the top of the Vantage Hill at Rye Grass Summit, much of the grass has been replaced by scraggly sagebrush. Or at least what’s left of them. Over the last 20 years, fires have ripped across different sections of Interstate 90 here including a big one last summer. The tall, once-white wind turbines found here looked kind of gray from the smoke.

The bridge across the Columbia River at Vantage has always been a transition point that says you are entering a very different environment. The terrain around the crossing gets only about 8 inches of rain a year in contrast to the 80 inches to 90 inches at the top of Snoqualmie a mere 100 miles to the west. This is classified as a true desert. The river’s elevation is only 660 feet, and its all uphill from here to Spokane in the next 150 miles.

I-90 rises out of the river valley along benches of basalt that mark the western edge of the Columbia plateau, each layer is a different lava flow. Once at the top of the grade, we begin a journey through the ancient flood channels left by the great Ice Age floods. Sagebrush is slowly replaced with grasses east of Moses Lake as we wind through the broad shallow channels of the floods. The scoured-out scablands around Sprague Lake remind us of the power of the floodwater.

East of Sprague, pine trees begin appearing on the horizon, first in stream valleys where water collects then as a solid wall about 30 miles out of Spokane. At this point we have risen a little under 1,500 feet in elevation from Vantage and gained 7 inches of annual precipitation; enough to give the pines the 15 inches they need to survive.

The journey ends as I-90 drops down the Sunset Hill, the eastern edge of the Columbia basalts.