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Alteration in Habitat Caused by Man Historic zoogeographic distribution patterns of Colorado fishes have been altered by man's activities. Since 1859, when the discovery of gold initiated the settlement of Colorado by large numbers of humans, the number of fish species and numbers of fish in many of the state's waters have been reduced. In some waters, this reduction has been so severe that no fish remain. Individual species have become rare, some disappeared entirely. Other species (native and introduced) which are tolerant of altered habitat con- ditions, have expanded in range and number. Industrial, domestic and agricul- tural activities have all contributed to the reduction of the state's fish fauna. Industrial effluents are varied in nature throughout Colorado. Chemical dis- charges from industrial sites may be toxic to fish if sufficient levels are introduced into a waterway. During the Colorado min- ing boom of the middle and late 1800's, no thought was given to the consequences of water pollution resulting from the min- ing industry. As a result, acidic, metal- containing effluents were dumped directly into hundreds of streams and rivers. To- day, many of the now inactive mine por- tals and tailings piles still introduce toxic levels of acid and metals to Colorado waters. Fish populations in many rivers are reduced or gone altogether. Some modern mines have million-dollar, advanced treatment plants that remove toxicants prior to discharge. Other discharges can be as unique as the industry involved. Cleaning com- pounds, petroleum products, photo- graphic chemicals, materials from metal- plating operations, ammonia, pesticides and herbicides have all been discharged into Colorado waters. Another possible byproduct of industrial activity is heated water. Hot water, or thermal discharge, can raise the temperatures of receiving waters to the point where fish cannot survive. or reproduce. All the causes described above eliminated some, or all, fish species from individual waters in the state. The impact of this pollution can be reversed through elimination of the source or through con- struction and operation of waste treat- ment plants. The impacted waters can be restocked. Native species can be used if donor populations survive in other areas. Urbanization of Colorado has also im- pacted the quality of the state's waters. Human sewage must be treated prior to discharge or the receiving water can become so foul that almost all aquatic life disappears. Over the years the treatment of human sewage has improved in Col- orado with the development of fairly sophisticated treatment facilities. Some problems remain. Chlorine, used to steril- ize sewage effluents, is toxic to fish in minute amounts. Ammonia is one of the end products of the purification of human nitrogenous waste products and, depend- ing on other conditions, can be toxic to fish. Ammonia and/or chlorine have eliminated fish populations from some river segments. In the plains of Colorado where arid conditions result in low flow conditions the volume of sewage dis- charged by a moderate-sized town can result in toxic situations below the effluent pipe. Even if toxic conditions are not created, the nitrogen and phosphorus compounds from the sewage treatment plants can enrich the streams. Algal growths can cover the stream bed. At night as the algal growths utilize oxygen for respiration, dissolved oxygen levels fall. Nightime oxygen levels can be so low that fish species are eliminated. If several communites discharge to a single river, nutrient enrichment often becomes a chronic problem. One such example is the South Platte River which receives treated domestic sewage from Denver, Greeley, Fort Collins and many other communities. Several species native to the South Platte mainstem are now restricted to foothills tributary streams upstream of sewage discharges. Species able to withstand moderate enrichment continue to be found in stretches of the river where water quality problems are not too severe. Surface water flows entering streams or rivers from urbanized areas is termed urban non-point runoff. These flows may be attributable to storm events, people over-watering lawns, car wash locations or other sources. Nutrients from fertilizers, spilled oil and gas, mud, silt, lead from automobile emissions are only part of the material that can be introduced into waterways. All such material can nega- tively impact fish populations. Some of the problems associated with agricultural activities are similar to those related to human sewage. Nitrogen and phosphorous fertilizers may reach streams and rivers causing nutrient enrichment. Species not tolerant of such enrichment disappear from affected streams. In some areas, cattle have been intensively grazed on land for decades. In moving to-and- from a stream for water, stream banks are pounded flat by thousands of cattle hooves. Undercut banks, aquatic and ter- restrial vegetation are eliminated. In many areas the riparian (streamside) vegetation has been removed to increase the amount of tillable land. Water temperatures increase in such areas since the shading provided by vegetation is no longer pres- ent. Increased erosion is a predictable result of plowing the land. Soil washed into waterways may settle to the bottom during the flow periods covering the stream substrate with silt. Also, irrigation return flows can introduce large amounts of silt and sediment. Turbidity of streams may increase, eliminating fish species that rely on sight to gather food. Many species discussed in this book require cool, clear, slow-flowing water and as this type of habitat has decreased, so have the populations of fish restricted to those environmental conditions. Conversely, a few species adapted to warm silty water have Colorado's Little Fish