A glacier might cling to a winter snow a hundred years and turn it to ice, a blue tool to rasp and pluck at rocks, before letting it go. Lingering summer snowfields might delay its passage for a time. But the water always wins at last, becoming, in one decisive instant, liquid again, and beginning its long journey to the sea. Plants and dry air will intercept some of its molecules, sending them back into the atmosphere to bloom as fog and cloud; but as rain, snow, or dew, these are soon commissioned to the land again. Water is so familiar to us that we seldom think about it. We know that fish swim in the lower lakes, and we are vaguely aware of the bewildering assortment of life-forms abounding in a pond. But life begins in the streams. Even cups of cold meltwaters, scooped out of a rivulet only a few meters away from its snowbank source, contain some life. Snow algae, which grow on the snowbank surface, often sufficiently dense to give the snow a distinctive red complexion, are released into the meltwater. In summer, small invertebrate life can be discovered in the standing pools of even the highest cirque. But conditions are not good for the development of complete aquatic food chains in the streams and lakes of higher elevations. Alpine lakes, or tarns, support little visible life. Often flanked by high ridges and peaks, many tarns receive scant direct sunlight during the day. Since these lakes occupy basins that capture tremendous amounts of snowfall, the snowbanks persist in the mountain shadows, and summer makes little progress in warming the water. Iceberg Lake, for example, is seldom free of floating ice, and its temperature never rises above 4° C in summer, even at the surface. Moving out of the cirque lakes, water is soon churning again, dashing downward many hundreds of meters to the valleys below, in rapids, cascades, and breathless waterfalls. Not surprisingly, few plants and animals are adapted to life in fast-moving water. Algae can be found covering streambed rocks and stranded, water-polished tree trunks. Securely attached by holdfasts, these small plant forms survive the rigorous stream flow that would destroy the larger vascular plants. Several species exist, from microscopic forms to branched filamentous algae whose long hairlike strands wave in the current. A surprising number of insects live on the stream bottom, finding a measure of protection from the current in the jumble of rocks. Underwater beetles live under the gravel or among the debris at the stream-edge, or cling to stones and sticks. Scurrying and creeping among the rock-crannies are the larvae of stoneflies, mayflies, and caddisflies. These and the small fish that venture up from lower lakes are the food of the water ouzel, a creature that loves the places where the waters thunder.