<h2>CHAPTER 1</h2><p><b>TheNATURAL SETTING</b></p><br><p>Colorado may be divided into three general physiographic regions: theGreat Plains on the east, the Colorado Plateau on the west, and theRocky Mountains in between (Fig. 1). Numerous mountain peaks exceed14,000 ft. (4,267 m) in elevation. Four major rivers have theirsources in the Colorado Cordillera. The Colorado River flows into thePacific Ocean (Gulf of California); the Rio Grande, South Platte, andArkansas Rivers drain to the Gulf of Mexico.</p><p>The topographic diversity and associated gradients in altitude resultin five major life zones. These life zones are best exemplified alongthe easternmost range of mountains in Colorado.</p><p>In central Colorado, timberline occurs at about 11,500 ft. (3,505m), above which lies the Alpine Zone. Protected cirques may containsmall glaciers, remnants of the neoglaciation about three thousand yearsago. Pristine lakes and clear brooks are set amidst fell-fields and meadowsrich in alpine wildflowers.</p><p>The Subalpine Zone is dominated by the spruce-fir forest (<i>Picea engelmannii–Abieslasiocarpa</i>), which occurs from 10,000 ft. (3,048 m) to timberline.</p><p>In the Montane Zone, from 8,000 to 10,000 ft. (2,438–3,048 m),Douglas fir (<i>Pseudotsuga menziesii</i>) predominates on north slopes andponderosa pine (<i>Pinus ponderosa</i>) occurs in more xeric locations. Largestands of aspen (<i>Populus tremuloides</i>) and lodgepole pine (<i>Pinus contorta</i>)also characterize this zone.</p><p>The Foothills Zone from 6,000 to 8,000 ft. (1,829–2,438 m) istypified by steep slopes and deep canyons. The Pleistocene glaciers thatsoftened the topography of the Montane Zone did not reach the foothills.Open forests of ponderosa pine occupy mesic sites and shrubsdominate dry slopes. Douglas fir occupies north-facing slopes and Coloradoblue spruce occurs in valleys.</p><p>The Plains Zone occupies elevations below 6,000 ft. (1,829 m),where grasses are the dominant vegetation of the Great Plains. Treessuch as cottonwoods, willows, and boxelder occur only along streamcourses.</p><p>There are eleven major drainage basins in Colorado, ten of whichoriginate in mountainous regions (Fig. 2). The Yampa, White, Dolores,and San Juan Rivers are subbasins of the Colorado River. The San LuisValley is a closed basin.</p><br><p><b>MOUNTAIN STREAMECOSYSTEMS</b></p><p>A variety of lotic (running water) habitats occur in the mountainousregions of Colorado. Unless otherwise specified, the term "mountainstream" is used broadly to include any lotic system in the western halfof Colorado, irrespective of size or elevation. More specific designations(e.g., brook or river) are described later in this section. However,there is an emphasis on the insect fauna characterizing relatively high-gradientrocky-bottomed stream reaches. The remainder of this sectionbriefly summarizes some of the attributes of mountain stream ecosystems.</p><p>For more detailed accounts see Leopold, Wolman, and Miller(1964), Hynes (1970a), Whitton (1975), Davies and Walker (1986),Ward (1992a and b), Williams and Feltmate (1992), Allan (1995),Leopold (1997), Waters (2000), and Wohl (2000).</p><br><p>SOME BASIC PRINCIPLES</p><p>Stream Order Analysis</p><p>Stream order (Strahler 1957) is a method of roughly classifyingrunning-water segments by size (Fig. 3). First-order streams are the head-watersources without tributaries. The confluence of two streams of thesame order produces the next highest order. Therefore, the joining oftwo first-order streams produces a second-order segment, two second-orderstreams produce a third-order stream, and so on. The MississippiRiver at its mouth is a twelfth-order stream. The largest rivers in Coloradoare eighth- or ninth-order. Although small in size, the lower-orderstreams are numerous. First- and second-order streams drain about 70percent of the United States.</p><br><p>Longitudinal Profiles</p><p>The longitudinal profile graphically describes a stream's gradient(slope) as a function of distance from the source. Typically, the longitudinalprofile is roughly concave (hyperbolic), with the highest gradient(steepest slope) in the headwaters and the lowest gradient in downstreamreaches (Fig. 4).</p><p>Although the hydrodynamic phenomena responsible for the concaveprofile need not concern us here (see Leopold, Wolman, and Miller1964), the downstream changes in ecological conditions are of interest.For example, the substrate of high-gradient headwater segments ischaracterized by coarse materials (boulders and rubble), whereass low-gradientdownstream reaches may be primarily sand and gravel.</p><br><p>Channel Patterns</p><p>There are three major types of channel patternss (Fig. 5). High-gradientmountain streams often flow in relatively straight channels.Even in straight reaches, however, the deepest portion of the stream(the thalweg) tends to moooove back and forth across the width...