Archive for May, 2010

The prairie town of Lethbridge in southern Alberta sits on a high plain just to the east of the Rockies. Winds from the west scream across the mountains, depositing on the peaks whatever moisture they might contain. Lethbridge, consequently, is bone dry, averaging about 15” of precipitation annually–about the same as Tucson, Arizona.

In regions as arid as this, life revolves around rivers. The St. Mary River flows out of Glacier National Park in the mountains of northwestern Montana, across the border into southern Alberta, just south of Lethbridge.

The river supports a lush corridor of trees, a rare green oasis bustling with wildlife. Cottonwoods and willows, uniquely suited to the wet and dry cycles of river flood plains, dominate the banks and are key species, providing food, shade and shelter for innumerable plants, fish, insects, amphibians, and animals.

Not surprisingly, the St. Mary River is also critically important to the human population, serving a range of uses including irrigation, hydroelectric power, and municipal water. Fed in large part by snow melt from the mountains, the St. Mary typically runs high in spring and lower in the summer. In 1951, a dam was built on the river to allow for limited storage of spring melt water for use later in the summer, as well as to facilitate a diversion of water to the adjacent Milk River basin.

Within a few decades it became apparent that cottonwood populations downstream of the dam were collapsing. Older trees were gradually dying and younger trees were not being established. The same pattern was evident in other arid western river basins below dam installations. Kayakers were among the first to notice the decline.

What was going on? Biologists at the University of Lethbridge, lead by Stewart Rood, wondered if the altered timing of stream flows, caused by the dam, were to blame. They compared historical records of stream flow data to the age, health, and species mix of trees along the river. They retreated into greenhouses and simulated the effects of different watering regimes on seedlings.

Newly hatched cottonwood seedlings are, in fact, perfectly adapted to the vagaries of life on the banks of arid western rivers.  To deal with the natural cycle of abundant spring water followed by long dry summers, the tiny trees, some no taller than inch or two in height, unfurl a few token leaves and then sink all of their energy into growing a single hair-like tap root.  As long as the trees can keep a toe in the dropping water table, they have a good chance for survival.

The building of the dam allowed for spring water flows to be terminated abruptly, rather than gradually as used to happen in the free flowing river. In other words, once the spring flood crest passed, dam operators shut spillway gates to save water for the drier summer months, resulting in an immediate and steep drop of the water table in the flood plain. The baby trees could not keep up and quickly withered.

In the greenhouse, Rood and his colleagues found that young seedlings could generate an impressive 2.5 cm per day of new root. They proposed that stream flow be reduced gradually enough to allow the trees’ roots to keep pace with the water table. Dam operators were willing to give it a try, provided there was ample water from spring storms or winter snow melt.

Heavy rain in June of 1995 offered a perfect opportunity for a trial. The test was wildly successful. By the end of the season the river banks were covered, for the first time in decades, with vivid green carpets of new cottonwood seedlings. Similar tests in other river basins yielded the same results. Because ideal conditions for new tree establishment in wild river systems occur about once every 5-15 years, there was no problem with resource managers implementing the additional spring water flows only in years with high spring precipitation or a large winter snow-pack.

This utterly elegant solution highlights the ongoing importance of continued investigation into the basic natural sciences such as biology, hydrology, and ecology. As natural systems are under increasing pressure, it is only our intimate understanding of how they function that will let us live in harmony with them rather than destroying them.

Yet I fear that many of the skills required to be a good student of the natural world are skills that have eroded with our changing culture. The speed of a calculator, for example, may allow us to quickly find answers and perhaps produce more, yet it also distances us from a feel for numbers that comes only from hours spent cranking through problems by hand.

Many folk that used to be able to rebuild every bolt in their cars now look under the hood with bafflement. Kids that once grew up intimately connected with the woods out the back door are now, instead, teaching their parents how to use computers.

While it is absolutely true that we must train people to work in hi-tech fields, with calculators, computers, and microchips, it is equally true that we still need to nurture tinkerers, careful observers, and those people who will float down rivers and first say, “What happened to all the trees?” and then, “I have an idea…”

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