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Wide Rivers Crossed
The South Platte and the Illinios of the American Prairie
By Ellen Wohl University Press of Colorado
Copyright © 2013 University Press of Colorado
All rights reserved.
ISBN: 978-1-60732-231-3
CHAPTER 1
At the Headwaters
Crossing the summit of an elevated and continuous range of rolling hills, on the afternoon of the 30th of June we found ourselves overlooking a broad and misty valley, where, about ten miles distant, and 1,000 feet below us, the South fork of the Platte was rolling magnificently along, swollen with the waters of the melting snows. It was in strong and refreshing contrast with the parched country from which we had just issued; and when, at night, the broad expanse of water grew indistinct, it almost seemed that we had pitched our tents on the shore of the sea.
— John Charles Frémont, on reaching the base of the Colorado Rockies after traveling westward across the Great Plains, June 1843
SNOWFALL
In much of the world, a flowing river represents the excess water that cannot be held by the plants and soil along the river's course. The adjacent landscape overflows into the river, each tributary swelling the flow of the mainstem. In contrast, the stream flow that sustains the largest rivers of the western prairie begins far from the dry lowlands, and tributaries heading on the prairie contribute little to the mainstem. This is one of the paradoxes of rivers of the western prairie: flowing for hundreds of kilometers across some of the continent's driest and most open country, the rivers begin in, and are sustained by, abundant winter snows falling in deep, narrow valleys of the topographic exclamation point that is the Rocky Mountains.
Snow starts to fall on the Rockies west of the prairie during September. In most years, the early snowfalls barely persist. A warm air mass moves eastward from the Pacific Ocean, and the thin skin of new snow melts into the soil or sublimates into the cold, dry air of 4,000 meters elevation. Air temperature in the drier central and southern parts of the Rockies can resemble a yo-yo, fluctuating rapidly up and down by 20°C or more from day to day. Within a month, however, at least a portion of each new snowfall remains on the ground. Great rivers and oceans of moist air flowing steadily inland from the Pacific collide with cold, dry Arctic air flowing down the spine of the Rockies. The tumultuous collisions create wind-driven snow granules and feathery powder snowflakes. What began as a light dusting of snow in September quickly deepens to a continuous covering of white as each new storm gradually builds the snowpack. The prairie remains a desiccated landscape of cured tan grasses, but in the mountains the snow can reach depths of 5 meters.
A year of abundant snowfall in the Colorado Rockies reflects the transfer of immense amounts of energy across half the planet. Equatorial and tropical latitudes receive the bulk of the solar radiation reaching Earth. Much of this intense low-latitude sunlight falls upon the broad expanse of the Pacific Ocean, creating a wide band of warm surface waters about the equator. The sun in a sense cooks the tropical oceans, warming the surface water and creating much higher rates of evaporation than occur over colder portions of the oceans. As water vaporizes and crosses the boundary from sea to air, warm, moist air billows up toward the sky. Some of this moisture cools, condenses, and falls back to the sea as torrential rains. Some of the moisture remains aloft and flows out from the equator toward each pole. Below these massive currents of air, warm water floats on the cooler, denser water beneath, flowing across the ocean surface toward higher latitudes until the water gradually cools and sinks to greater depths. These surface currents transfer heat to the atmosphere before sinking into the cold darkness of the deep ocean and returning at depth toward the equator. This is part of the aptly named Great Ocean Conveyor Belt, an endless cycling between the Pacific and the Atlantic that carries heat up into the North Atlantic. Only because of this pattern is northwestern Europe warmer and more suited for agriculture and habitation than equivalent latitudes in Canada and Siberia.
Most of the water vapor carried by air moving toward the poles from the equatorial oceans falls as precipitation before the air reaches 30° North and South. This is where the spent, dry air descends back toward Earth's surface before continuing at low elevations within the atmosphere back to the equator. The complete cycle is known as the Hadley Cell after the eighteenth-century gentleman who first described it. Unless another moisture source such as an ocean with warm surface water is nearby, drylands — prairie, pampas, veld, steppe, savanna, and desert — occupy continental interiors at 30°–40° latitude.
This latitudinal belt accounts for a big chunk of the Southern Rocky Mountains within the United States. Every other mountain range between the Pacific and the Rockies only exacerbates the dryness. The ocean of moist air flowing eastward from the Pacific rides a topographic roller coaster, rising and cooling over each mountain range and dropping more of its precious moisture with each rise. By the time the air reaches the eastern half of the Rockies, there often isn't much water vapor left. These mountains receive snowfall each year only because of their great height. Most winds that flow eastward down the mountain front in winter come as warm, dry chinooks that rearrange anything portable in the landscape but do nothing to increase precipitation on the plains. This is a second paradox of the rivers of the western prairie. The Rockies make the prairie drier than it might otherwise be, but they also supply the major rivers of the prairie. These rivers are able to flow through much, if not all, of the year only because of the snow that falls and then gradually melts off the Rockies.
Air does not flow through the Hadley Cell as regularly as the hands of a clock making a circuit, however, and blips in the circulation pattern can create years of heavy snowfall in the Colorado Rockies. While warm surface waters flow toward the poles and cold water flows back toward the equator at depth, water also moves across the Pacific in east-west currents. Cold water wells up from the great depths off the west coast of South America and then flows west across the surface of the tropical and equatorial Pacific, warming as it moves and creating a persistent pool of warm water around Indonesia and northern Australia. Warm water means evaporation and heavy rainfall. Monsoon rains fall on the lands around the western Pacific from December to February during most years, supporting lush rainforests, while the Atacama and Sechin Deserts lie at the other end of the Pacific.
Every few years, for reasons still unknown, the entire pattern reverses. Sea level pressure, which is normally low over the western Pacific and high over the eastern Pacific, flips in a pattern known as the Southern Oscillation. Warm surface waters slosh back toward the eastern Pacific. Indonesia goes into drought, and western South America receives torrential El Niño rains during the Christmas season. The effects of the combined El Niño–Southern Oscillation (ENSO) spread from the tropical Pacific like a rock thrown into a still pond. Normal rainfall and snow patterns change from southern Australia to India, and more abundant winter and spring precipitation delights skiers and water managers in the Colorado Rockies.
Year-to-year variations in snowfall across the Rockies also reflect the Pacific Decadal Oscillation (PDO). The northern Pacific Ocean oscillates between warmer and cooler conditions at time spans of twenty to thirty years. Cool phases of the oscillation correspond to drought in Colorado as the surface of the Pacific cools off western North America. The cooling ocean reduces evaporation and inland transport of moisture. The jet stream is the express freight bringing much of this moisture inland, and changes in sea level pressure occurring during the PDO act like a giant switch, sending the jet stream further north. The last warm phase of the PDO persisted from 1977 to 1999, but during the past decade the PDO has alternated at shorter intervals between warm and cool phases.
Between the ENSO, the PDO, and other, less regular fluctuations, snowfall in the Rockies can bury the mountain meadows so deep that not a slight dimple reveals the little headwater creeks, or it can be so miserly that in midwinter the creeks still flow freely between whitened banks. These fluctuations in snowfall translate all the way downstream to the dry plains.
SNOWMELT
A lot happens between a winter storm over the Rockies and stream flow in the rivers of the western prairie. First there is the snow itself, a much more complex entity than delicate little six-pointed flakes drifting quietly down. As a result of variations in the moisture content of the cloud in which the snowflake formed, air temperature when the snow fell, and wind speed at the surface on which the snow landed, not all snowflakes are created equal. Snow falling during the first part of the winter in the Rockies tends to be the light, fluffy "powder" snow that delights skiers. This is the low-density snow that seems bottomless when you fall into it and punch your ski pole down looking for support to get back up. As air temperatures grow warmer during March and April and strong winds buffeting the mountains break snowflakes into fragments that can pack together tightly, falling snow compacts into a dense, wet mass detested by homeowners shoveling their driveways.
The density of the snowpack exerts a vital control on how much water will actually melt out of the pack, as does the depth of snow. Depth is easy to measure, except that it varies quite a lot across the steep, wind-blasted topography of the Rockies. Density is more difficult to measure because it continues to alter once the snow has fallen. As the snow keeps coming — 1 meter, 2 meters, 3 meters, 4 meters — the overlying weight compacts the snow at the base of the pack. In the dark, cold depths, a subtle alchemy occurs as delicate flakes with projecting points melt slightly under pressure and then re-freeze as larger, more spherical particles of greater density. Some of the re-freezing snow-water creates bridges between adjacent particles, further reducing the air spaces within the snowpack. Density of the snowpack progressively increases as snow accumulates, but the rate and degree of this increase vary. Sometimes layers of solid ice form within the pack through the combined effect of settling, melting, and re-freezing. Then again, extremely cold temperatures and relatively shallow snow depths can result in relatively little change in snowpack density.
Snow has historically fallen during every month of the year in the Colorado Rockies. Spring typically begins with a series of "false starts" when snowstorms follow periods of warmth. Eventually, even the densest snowpack warms under the influence of increasing air temperatures and lengthening hours of sunlight. Feathery snow crystals and dense ice lenses vanish during the next few weeks as the snowpack disintegrates. Snow sublimates directly into water vapor as dry winds slip across the surface. Given the gusts that can blast across the Rockies and plains in springtime, it is surprising that any snow remains to melt into water. Yet some of the snow does melt at the surface before filtering down into the porous snow beneath or flowing across the surface crust of ice. Meanwhile, snow at depth melts and seeps downward. As melting increases, the snowpack behaves like a sponge, storing the melt water until it finally becomes saturated and water begins to percolate downward. Arriving at the base of the snowpack, some of the melt water infiltrates into the soil. Some of it accumulates to form a saturated zone that moves downslope toward a stream channel. All across the highlands the ground becomes mushy with water moving down toward the valleys.
This is one of those recurrent miracles we take for granted: water filtering drop by drop into layers of sand and gravel in the soil through which it can seep or finding minute cracks in the seemingly impenetrable bedrock, forced onward by the pressure of the water behind it. Water following shallower underground paths reaches the stream channels in days to weeks, but some of the snowmelt filters down to the saturated zone below the water table, moving so slowly that it reaches the nearest stream channels months or years later. All of these millions of tiny, hidden pathways gradually fill up with water as spring continues into early summer, until the stream banks cut into mountain meadows dribble water like a leaky faucet.
Melting of the snowpack is largely invisible on the slopes while the snow is present. Early hikers may "posthole" at every step as they break through the icy surface crust and sink into the mushy snow beneath, but the only real indication of the progressive melting is the gradual rise in stream flow and the slow reemergence of snow-buried rocks and fallen logs. The most intriguing signs of all the action occurring in the snowpack appear once the melting is largely complete. Small, linear mounds of soil lie scattered across the slopes, discontinuous hieroglyphics written by rodents tunneling between their burrow entrances at the base of the snowpack. Melt water flowing downslope carries the sand and gravel dug up by the little animals, and the tunnels fill with sediment that remains briefly after the snow is gone. A few summer rains or the heavy tread of a passing elk and the perfect casts of tunnels smear into loose heaps of sediment.
Even without the mounds of pocket gophers, a mountain hill slope is not a smooth, regular surface. Snowmelt flowing across the recently thawed ground finds every little hollow and trough, concentrating into small rivulets in which water a few centimeters deep starts to erode exposed sediment grains. Occasionally, the soggy hill slopes give way abruptly in debris flows or landslides that move masses of sediment downslope. The winter streams of clear water flowing quietly patterned in green and black against the white ice give way to churning masses of milky brown color. Emily Dickinson described this transformation: "When the snows come hurrying from the hills, and the bridges often go." For most of the year, streams in the Colorado Rockies carry only a few milligrams of sediment in suspension per liter of water. During snowmelt this level can rise to more than a thousand milligrams per liter, although mostly remaining below a hundred milligrams.
The mountain tributaries of the South Platte River spread widely from north to south along the spine of the Rockies, collecting melting snow like an enormous rake. Up near the Wyoming border, the Cache la Poudre River starts in a modest lake in Rocky Mountain National Park and flows northeast before curving eastward to join the South Platte near the city of Greeley. Named by French fur trappers for a cache of gunpowder left near the channel, the Poudre was Colorado's first federally designated wild and scenic river. Only a few kilometers from Poudre Lake, the headwaters of the Big Thompson River flow southeast through Forest Canyon, one of the most rugged backcountry areas in the national park, collecting water from cirque lakes named Azure, Inkwell, Lonesome, Rainbow, and Highest Lakes before funneling into the narrow gorge of Big Thompson Canyon and joining the South Platte on the plains. North, Middle, and South St. Vrain Creeks drain the mountains south of Longs Peak. Like the Poudre, the names of these rivers reflect the history of beaver trapping. Both David Thompson and Ceran St. Vrain trapped beaver in the region during the early decades of the nineteenth century. South of the St. Vrain drainage lie Boulder Creek, Clear Creek, Tarryall Creek, and the forks of the South Platte itself. The South Fork of the South Platte lies close to the northern headwaters of the Arkansas River, the next major river of the western prairie, just as the northern tributaries of the Poudre River lie close to the southern headwaters of the North Platte River. The abundance of melting snow flowing down steep slopes has carved the landscape into a dense network of streams, where only relatively narrow topographic high points separate adjacent drainages.
Spring snowmelt across the broad upper basin of the South Platte is a flush that spreads upward with time. As the nearby snow melts, stream flow at the lower elevations rises and then remains high while tributaries farther up the drainage basin swell with melting snow at the higher elevations. The result is a remarkable synchronicity in the timing of peak flow throughout the mountains. From streams near the top of the mountains at elevations of nearly 3,000 meters down to streams flowing beyond the mountain front at 1,400 meters elevation, peak flow commonly occurs between June 8 and 16 each year.
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Excerpted from Wide Rivers Crossed by Ellen Wohl. Copyright © 2013 University Press of Colorado. Excerpted by permission of University Press of Colorado.
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