Beavers are herbivores that construct dams and canals along rivers with access to aspen or willow trees, perennial flow, channel gradient < 15%, and valley width on the order of 45 m (Allen, 1983). On such streams, beaver populations average 2-3 colonies (6-40 animals) per km of river. Beavers create longitudinal steps along channels by building and maintaining dams of wood and sediment. Water is ponded upstream from the dam, slowing the passage of flood waves, storing sediment and nutrients, and creating germination sites for aquatic and riparian vegetation as the pond gradually fills with sediment (Butler and Malanson, 1995; Gurnell, 1998) . Beaver dams thus increase the habitat diversity and stability of streams and valley bottoms. Although scientists did not document the effects of early 19th century beaver removal in the rivers of the Colorado Front Range, it is reasonable to assume that: flood peaks became at least slightly shorter in duration, with reduced overbank flooding and groundwater recharge along the valley bottom; bed and bank erosion and sediment mobility increased, possibly leading to preferential filling of pools and sedimentation of spawning gravels along riffles; and rivers became a little less stable and diverse (Naiman et al., 1986). Beavers were trapped along all the rivers of the Front Range during the early 19th century (Wohl, 2001).
(Photo by E. Wohl)
Mining in the Front Range included placer mining (alluvial placers are pieces of metal ore dispersed through sediments deposited by water or glacial ice) and lode mining (the metal is dispersed in small veins throughout bedrock outcrops). In both cases, miners sought to concentrate the ore. Placer miners used rocker boxes. A rocker box is a wooden crate with a sieve at the bottom and a ridged shoot beneath. Sediment and water are dumped into the crate and the mixture is then rocked. Water is run down a sluice (a long, narrow wooden box with washboard-style grooves) to separate the denser metals from the surrounding rock fragments. In hydraulicking or hydraulic mining, water under pressure is used to break apart valley-bottom sediments. Placer miners also used dredge boats from the end of the 1890s into the 1950s. Buckets on a conveyor belt scoop up channel sediments, which are then run through sluice on the dredge boats, or chemically processed using amalgamation with mercury. Two people operating a rocker box or hydraulic system, using 400-3,000 liters of water, could process 2-4 cubic m of sediment in 10 hours; a dredge boat could process 6,000-6,600 cubic m of sediment in 10 hours (Silva, 1986). Miners commonly worked the valley-bottom sediments down to the bedrock contact and out to the valley margins.
(Photograph courtesy of Western History Department, Denver Public Library)
Placer mining severely disrupted the coarse-surface layer that helps to stabilize many rivers in the Front Range. This resulted in dramatically increased sediment loads, increased turbidity, loss of pools, and increased lateral movement of channels (Hilmes and Wohl, 1995). The mercury used to process placer metals continues to be a source of toxic contamination in streams a century later.
(Photograph courtesy of Colorado Historical Society)
Lode mining resulted in further increases in sediment entering rivers from tailings piles and from newly deforested hillslopes that became more subject to debris flows and landslides. The net effect of both placer and lode mining was to overwhelm the transport capacity of adjacent streams, resulting in sedimentation, loss of habitat diversity, and channel stability. The Clear Creek drainage was the most heavily mined area of the Front Range, but the Boulder Creek drainage, the Middle Fork of the South Platte, and portions of the Poudre River drainage were also affected (Wohl, 2001).
Placer miners needed water to work their claims, and some of the earliest water diversions in the Front Range occurred during the early 1860s in association with mining. Water diversion and storage increased rapidly as regional population increased and agricultural communities were established along the eastern base of the Front Range during the 1860s and 1870s. The first dam in the region was the 1869 reservoir on Ralston Creek in the Clear Creek drainage, and the first diversion across the Continental Divide was the 1882 Cameron Pass Ditch (Wohl, 2001). The doctrine of prior appropriation was incorporated into the 1876 Colorado Constitution. At present, none of the major rivers within the Front Range are free-flowing for their entire length within the mountains. North St. Vrain Creek is one of the least regulated; it is not dammed until Buttonrock Reservoir near the base of the mountains.
(Photo by E. Wohl)
Flow regulation alters the magnitude and timing of flow. This can alter sediment transport and channel morphology (Ryan, 1997), as well as aquatic and riparian communities (Merritt and Wohl, 2006; McCarthy, 2008). Where flow volume and peak flow magnitude are reduced, channels can become narrower, and aquatic and riparian organisms become less diverse and abundant.
Extensive and intensive timber harvest closely followed mining in the Front Range. Wood was needed for rocker boxes, mine timbers, buildings, firewood, charcoal, and railroad ties. Much of the timber went to form ties for newly built railroads bringing supplies and people to the mining centers and carrying ore down to cities at the base of the mountains.
(Photograph courtesy of American Heritage Center, University of Wyoming.)
Channels were modified to facilitate the passage of ties: overbank areas were blocked off and obstructions within the channel were blasted out. The passage of huge masses of logs also acted like a giant scouring brush, altering channel morphology and riparian communities. Although tie drives seldom lasted more than a few years on any river, and more than a century has passed since the last tie drive, Young et al. (1989) found that rivers with tie drives were wider, had less riparian or bank cover for fish, lower habitat diversity, less wood, smaller pools, and less diverse riparian vegetation than otherwise similar channels that had not experienced tie drives.
(Photo by E. Wohl)
The first transcontinental railroad reached Cheyenne, Wyoming in 1867 and a branch connected to Denver in 1870. Other lines soon followed. These lines stimulated development of narrow-gauge lines to the mining centers in the mountains. Railroads needed lumber for ties and charcoal to fuel steam engines, and numerous sawmills were built in the mountains to satisfy this demand (Wohl, 2001).
Roads were built simultaneously with railroads in much of the Front Range during the latter half of the 19th century. Barely cleared tracks gave way to corduroy roads on which logs were laid side by side to reduce the effects of mud and boulders, and then to graded and paved roads. Like railroads, roads can reduce the width of the riparian corridor by occupying part of the valley bottom.
Sediment eroded from the road surface or the road embankment, or hillslope instability triggered by compaction associated with the road, also affect adjacent streams. Contaminants and traction sand and gravel carried from roads by runoff enter rivers. Lorch (1998) estimated that traction sand from Interstate 70 introduces 150 times the natural sediment yield to adjacent Black Gore Creek, burying riparian vegetation and filling pools. North St. Vrain Creek is today the only major Front Range river that is not closely paralleled by a road, although parts of other rivers (e.g., upper Big Thompson & upper Poudre Rivers) do not have adjacent roads.
(Photo by E. Wohl)
Historical recreational effects on rivers of the Front Range were associated mainly with fishing. Trout in particular declined in abundance as a result of the combined effects of intense fishing to supply markets in Denver and in mountain communities, and the channel changes associated with mining, flow regulation, timber harvest and tie drives, and roads and railroads. With the exception of the native greenback cutthroat trout (Oncorhynchus clarki stomias) now present only in headwaters with barriers such as waterfalls, all other species of trout present in the Front Range are exotics introduced from elsewhere.
Fishing continues as a contemporary recreational effect on rivers of the Front Range, along with commercial whitewater rafting. Tens of thousands of people now take day trips on the Poudre River each summer, with put-in and take-out at a few spots along the river. These intensively used sites experience soil compaction, bank erosion, and removal of riparian vegetation.