Prairie
Page 18
Then two apparently unrelated events occurred. In 1985, as drought took hold of the prairies in earnest, the U.S. government introduced a program designed to prevent soil erosion and support the region’s beleaguered farmers. Known as the Conservation Reserve Program, or crp, this initiative provided an annual payment, on a 10- or 15-year term, to landowners who agreed to retire unproductive land and plant it to grass, alfalfa, or other forms of soil-binding permanent cover. Conceived as a temporary response to hard times, the crp was subsequently renewed, updated, and expanded to buy protection for thousands of fragments of upland habitat in the Dakotas and adjacent parts of the Pothole Region. In so doing, the program almost inadvertently created a vast new resource of quality upland habitat for wild animals and birds. With better cover for their nests, ducks on crp lands frequently enjoyed success rates in the range of 20 to 30 percent, more than enough to lift the populations out of their slump. (Despite this quacking dividend, the Conservation Reserve Program has recently been curtailed, and millions of acres are expended to be returned to cultivation in the next few years.)
Meanwhile, at the same time as new habitat was being created, ducks received another boost when a decade of drought finally drew to a close. When the long-awaited rains returned in the early 1990s and the sloughs filled up, the Prairie Pothole Region moved from drawdown to regeneration in a headlong rush. After a period when it had seemed that ducks were on their way down, they were suddenly present in every ditch and slough, and the populations of several species surged toward record numbers. For the first time in history, however, the pulse that replenished the population came mainly from the crp lands in the Dakotas, rather than from the Canadian prairies, which had been the powerhouse of duck production in previous surveys. This development was widely seen as a warning that the recovery was precarious, especially in districts where large tracts of land were under cultivation. Yet for all the stresses and strains of the previous century, the ability of pothole-nesting ducks to rebound from near-disaster stands as a testament to the regenerative powers of prairie wetlands and the seemingly inexhaustible resilience of the wet-dry cycle.
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> QUEER DUCKS
In his Life Histories of Familiar North American Birds, pioneering ornithologist Arthur Cleveland Bent described an unusual sighting of a northern pintail. “Once,” he wrote, “while crossing a tract of burned prairie [in the early 1900s], I saw a dark object fully half a mile away, which on closer inspection proved to be a pintail sitting on a nest full of half roasted eggs; this was a beautiful illustration of parental devotion and showed that the bird was not dependent on concealment.”
The northern pintail is quite literally a queer duck. A member of the dabbling duck tribe, it looks a bit like a mallard that has been stretched at both ends to produce an elongated neck and, in the male, an extravagant, quill-like tail. The drakes are also distinguished by a sleek chocolate-brown head, a pure white breast, and a swirled boundary where the colors meet on the neck. (If an Art Deco designer had set out to create a duck, it might well have resembled a male pintail.) Hens, are clad in camouflage plumage the color of dry grass, plain but elegant.
What makes northern pintails unusual is their lackadaisical approach to the selection of nesting sites. Unlike most other dabblers, which conceal their nests in thickets of grass or shrubs, pintails settle down wherever the spirit moves them, even in sites that are completely open. Under natural conditions, when their nests were mere points in unbroken vistas of grass, this strategy presumably made some kind of sense. A predator had to be lucky to find them. But these days, when the grasslands are broken into fields and fragments and liberally studded with trees, hawks and other predators have gained a troubling advantage.
Pintails that nest in crop stubble are at special risk. A recent study of nest success in an intensively farmed area of southern Saskatchewan determined that out of every hundred stubble-field nests, no more than four were successful. The reasons for this abysmal record were spring seeding, which destroyed 20 to 33 percent of the nests, and high levels of predation, which took out the rest. (Nests in denser cover enjoyed greater success, at 6 to 37 percent.) As a result, pintails are the only species of predominantly pothole-nesting ducks that have not rebounded from the 1980s drought. “Parental devotion” has not been enough. But the people at Ducks Unlimited hope that a shift in agricultural practices toward the planting of fall-seeded, or winter, wheat may give pintails a better chance by reducing spring disturbance and providing early-season cover for nests.
Northern pintails
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Prairie Rivers
Prairie potholes and other ephemeral wetlands are classic examples of ecosystems adapted to cope with almost constant disturbance. Faced with a relentlessly dynamic environment, pond life has become almost as fluid and responsive as water itself. And this same description also applies, though in strikingly different ways, to the life of the two major river systems that cut across the Great Plains. They are, to the north, the Saskatchewan and, to the south, the Missouri.
The Saskatchewan system extends across the Canadian plains like a spindly arm, with its fingertips in the icefields of the Alberta Rockies and its elbow at the north end of Lake Winnipeg. Rising as a thousand milky meltwater trickles that tumble down the continental divide, the water converges into creeks and then rivers that flow to the north and west and ultimately join to form the North Saskatchewan River. Meanwhile, water cascades down the mountains to the south and west into the valleys of the Oldman, Bow, and Red Deer rivers; together, they become the lovely, often lazy South Saskatchewan River. Cutting cross-country through the farmlands of Alberta and Saskatchewan, the two branches meet east of Prince Albert to form a single, gray-green flow, the Saskatchewan River. From there, it is on to Lake Winnipeg and then north, via the Nelson River, into the Arctic Ocean at Hudson Bay. From headwaters to mouth, the major rivers of this system cover a combined distance of some 1,600 miles (2,600 kilometers) and provide drainage to 400,000 square miles (or more than a million square kilometers) of land in the three Prairie provinces.
The Missouri River system is, if anything, even more magnificent. From sources in the Bitterroot Range of northwestern Wyoming and southwestern Montana, the mainstream of the Missouri meanders east across Montana and then south through the Dakotas before emptying its muddy flow into the Mississippi River at St. Louis, for a total run of almost 2,500 miles (4,000 kilometers). Routed along the shore of a long-vanished Ice Age glacier, the great river snakes diagonally across the center of the continent, attracting drainage from both the north (through tributaries such as the Milk and the Frenchman) and the west (through, among others, the Yellowstone, Little Missouri, Niobrara, and Platte rivers). All told, the rivers of the Missouri system drain an area of about 530,000 square miles (1.4 million square kilometers), including the southernmost Canadian prairies and one-sixth of the continental United States. That’s the entire sweep of country from the Rockies to northeastern Missouri and from southern Saskatchewan to central Kansas.
From central Kansas south, by contrast, the land slopes ever so gradually away from the Missouri basin and entrains the rivers to run directly into the Mississippi River or the Gulf of Mexico. These include the Cimarron, Canadian, and Arkansas rivers of the Arkansas River system, and the Brazos River of central Texas. Thus, in one way or another, the waters of the dry-and-dusty American heartland all end up in the Caribbean Sea and, thence, in the North Atlantic.
Of Fish and Flood
Although we tend to think of rivers as if they were all alike—just an onward rush of liquid confined between banks—every watercourse is unique, distinguished by a particular set of physical characteristics. Does the water freeze solid in winter or flow freely throughout the year? Is it fast and deep or soupy and shallow? Clear as crystal or burdened with eroded soil? Does it shrivel away to nothing in midsummer? The answers to these questions define the conditions of life in a particular stream and determine the range o
f species adapted to survive in it. Thus, rivers like the North and South Saskatchewan and upper Missouri, which are ice-bound for part of the year, support far fewer types of fish (some two or three dozen at most) than rivers farther south, like the Arkansas and lower Missouri (each with a species list of 100-plus). Similarly, the seasonally low and often oxygen-deficient waters of the arid western plains tend to have fewer types of fish than the more generous and stable rivers of the tall-grass prairies. In the eastward-flowing Arkansas River, for example, the species count more than doubles from west to east, rising from 54 to 117.
Autumn works its magic along the banks of the South Saskatchewan River, near Saskatoon, Saskatchewan. Tragically, the river is in failing health, due in part to an 84 percent reduction in summer flows in the past century.
Arthur Savage photo
A river is not really one place but many: a long, sinuous linkage of different environments connected by the flowing fabric of the water. Even adjacent stretches of the same river can be remarkably dissimilar, from sparkling, well-oxygenated riffles where the water slips over gravel beds to slow, swirling pools behind logjams to marshy backwaters set away from the main channel. Here fish spawn or feed or seek refuge from predators. There frogs lay their eggs, turtles bask in the sun, and great blue herons stalk through the shallows. The diversity of life in the river depends on the ability of the water and its basin to maintain a constant supply, or flow, of different habitats, so that a diversity of new opportunities is continually created.
The temperature shock of winter is just one of the variables that define the character of a stream or river.
Arthur Savage photo
Painted turtle
For the major rivers in both the Saskatchewan and Missouri systems, the principal force of renewal has, until quite recently at least, been the chaotic ritual of spring flooding. As so-called large floodplain rivers, the North and South Saskatchewan and the Missouri are naturally characterized by highly dynamic flows and were never meant to be confined within narrow channels. In the case of the Missouri, to cite the best-documented example, the river followed a seasonal rhythm, falling to its lowest levels in winter, experiencing a small surge at breakup in spring, and then, depending on the year, often really kicking over the traces in early June. Forced by the onrush of snowmelt from the mountains, the river swirled up over its banks and out across its wide, flat-bottomed overflow valley, or floodplain. With the full force of a natural disaster, the water scoured away sandbars, uprooted shrubbery and full-grown trees, tore out logjams, and generally ran amuck, leaving the entire floodplain in chaos.
Northern pike
But all that mess and disorder held the seeds of rebirth. When the flood subsided, it revealed a soggy landscape of ponds and puddles, many of them linked to the river’s main flow, that would quickly grow up into a lush and varied mosaic of oxbow lakes, marshes, and wet meadows. Amphibians laid their eggs in these quiet waters; certain fish, like the northern pike, splashed through the shallows to spawn; others relied on the floodplains as rearing grounds for their young. And all this productivity was accelerated by an input of nutrients from the rotting muck that was strewn around the countryside in the wake of the flood. In this way, a landscape of aging floodplain forests and silted-up wetlands was periodically restored to the freshness and vigor of youth, and the resources of the system were freed up to support a new burst of growth.
During the winter, the northern leopard frog often hibernates on the muddy bottom of a creek, river, or lake— virtually motionless— then pops back up to the surface when spring comes. Like many other species of frogs around the world, the northern leopard frog has suffered recent, severe declines in population, probably the combined result of habitat loss, disease, pollution, and climate change.
Meanwhile, as the main flow of the river fell back into its channel (around many a new twist and bend), its waters were muddied with silt and decaying vegetation. Yet this apparent burden constituted a food-rich resource for the community of plankton in the river and hence for plankton eaters, such as insect larvae, crustaceans, and fish, and ultimately for most other organisms that lived in the stream. This surge of productivity is known as the flood-pulse, and, though still poorly understood, it appears to be as important to the life of rivers as the wet-dry cycle is to marshes. For example, many species of river fish—including native prairie minnows like the Arkansas River shiner and speckled chub—rely on spring flooding as a signal to spawn. By responding to this seasonal cue, the fish are able to coordinate their life cycle with the turning of the year, thereby ensuring that their young receive the full benefit of both the flood-pulse of productivity and the growth-enhancing warmth of midsummer.
Johnnydarter
Green sunfish
In the last half century, however, this natural rhythm has been disturbed. In some instances, the seasonal flood cycle has been dampened by an overall lowering of the rivers’ flow caused by heavy withdrawals for irrigation. (These demands have intensified since World War ii, when pumps powered by surplus airplane engines were first hooked up to central-pivot irrigation systems.) As water is pumped out of underground formations, the level of the groundwater falls, just as if it were being sucked out by thousands of high-tech drinking straws. As the water table drops, there are no reserves left near the surface to trickle into the river, through seeps and springs, and help to keep its flow replenished. As a result, the river is now so diminished that it can no longer overflow, flood the surrounding plain, and replenish the groundwater through a trickle-down effect. Caught in this self-defeating cycle, the water table underlying much of the southern plains dropped by anywhere from 10 to more than 100 feet (3 to more than 30 meters) between 1940 and 1981, and the flow of many rivers was also drastically curtailed. To cite one poignant example, the Kansas stretches of the Arkansas River have dwindled from an average flow of 49 cubic feet per second, pre-1960, to a mere 7 cubic feet per second in 2005, a decrease of 86 percent.
Not surprisingly, this degradation has taken a heavy toll on species that are adapted to live in free-flowing, seasonally flooding rivers. A century or so ago, that description fit most rivers on the southern plains, including the little Smoky Hill River of west-central Kansas. When the fish of the Smoky Hill basin were first catalogued in 1885, researchers hauled in specimens by the dozens. The list of species was evocative: hornyhead chubs, johnnydarters, brassy and suckermouth minnows, black bullheads, stonecats, green sunfish, black-nose shiners, southern redbelly dace, and many others. In 1985, when the survey was repeated, all the above-named species were gone, a change attributed to reduced water levels, loss of habitats, and the suppression of the flood-pulse cycle. In their place were a relatively small number of more-or-less ubiquitous introduced fishes, including largemouth bass and carp, that were able to cope with the new conditions.
Often found in swampy margins along rivers and creeks, the great blue heron uses its bill not only for preening but also for nabbing slippery frogs and fish.
The withdrawal of groundwater for irrigation is not the only postwar project that has left its mark on the ecological integrity of prairie rivers. Since the 1950s, engineers have also been busy constructing dams, diversions, impoundments, and other impediments to ensure that water is made available for human purposes. Across the entire Great Plains, with its thirsty cities and farms, there is scarcely a stream of any significance that has not suffered some such “improvement.” For instance, the North Saskatchewan River is held back by two major impoundments before it even reaches the plains, pooling out to form Abraham Lake (in the mountains west of Red Deer) and the Brazeau Reservoir (southwest of Edmonton). The South Saskatchewan River is backed up by the Gardiner Dam (south of Saskatoon) at the head of the long noodlelike contours of Lake Diefenbaker. And this is to say nothing of the dozens of small reservoirs on dozens of tributaries that affect the movement of water through the Saskatchewan River system.
Then there’s the Missouri. Now more accurately d
escribed as a string of reservoirs than as a river, the upper Missouri is blocked by seven major dams, all but one of them built in the immediate postwar period. Working our way downstream from the river’s headwaters, they are: Canyon Ferry (1952) and Fort Peck dams (1940) in Montana; Garrison (1953) in North Dakota; Oahe (1958), Big Bend (1963), and Fort Randall (1954) in South Dakota; and Gavins Point (1955), at Yankton, near the South Dakota/Nebraska border. Between Sioux City and St. Louis, by contrast, the lower Missouri has been intensively reengineered for a different use. Instead of being interrupted by dams, this “home stretch” of the river has been dredged, deepened, and banked to convert it into a fast-flowing, uninterrupted channel for barges. Together, these alterations have been admirably effective at meeting their stated goals, whether navigation, power generation, recreation, or flood control. But they have also— to an extent that seems inevitable with hindsight but that was initially unforeseen—drastically altered the life of the Missouri.
The whole idea of damming rivers is to regulate seasonal variations in flow and make a chaotic natural system more predictable. Rather than permit spring meltwaters to rush out over the floodplain—by now a complex of cornfields, multilane freeways, and urban sprawl—engineers can hold back the flood behind massive barriers. With the exception of freak seasons like the spring of 1993 (when rapid snowmelt and heavy rains combined to overwhelm the system and create havoc throughout much of the Missouri basin), the creative tumult of spring flooding is over and done with. And it is not just the magnitude of the peak flows that is now under human control: their natural rhythm has also been altered. Under modern conditions, the volume of water that is permitted to pass through the sluicegates depends not on the logic of the seasons but on human demand. As a result, the more-or-less free-flowing stretches of the Missouri (between and below the dams) now typically experience their peaks not in spring but in winter, when the draw on the system is low, and fall to their lowest ebb during the growing season. This change completely confounds the flood pulse on which so many river-adapted organisms depend and makes the river a less livable environment for them.