Environmental geology is a broad science, concerned not only with the location of cities and towns, but with the uses people make of the land and its economic products, and with the relationship between the geological character of the land and the present and future location of roads, dams, bridges, factories, homes, recreation facilities, sanitary land fills, and even sewage plants. Two aspects of environmental geology which are particularly pertinent to Colorado’s residents are discussed below. _Landslides_ and slumping rock or earth are a frequent menace to Colorado’s development in the Mountain Province. Often activated by heavy rains or deep manmade cuts, they can cause—and _have_ caused—much damage to roads, buildings, and other works of man. The flanks of North and South Table Mountains, near Golden, are mantled by thick landslide debris; intermittent movement of the individual slides has repeatedly affected the railroad, irrigation ditches, and roads. As many as six different slides have moved within a single year. In one slide area, asphalt road material is estimated to be thirteen feet thick; successive layers of pavement have been laid one on top of another to keep the street up to grade. Landslides and landslide-prone areas may not be obvious to the untrained eye. Each year buildings and roads are constructed on unsuitable rock and soil foundations, in places where some degree of land slip is almost inevitable. Building in such areas is risky, but sometimes worth the risk; if condition are less than ideal, risks can be reduced by specialized types of construction. _Floods_ are a perennial threat to much of the state, because of the high relief of the drainage basins and the torrential nature of the spring and summer rainfall. Their damaging effects were realized early in Colorado’s history, when canyons were used as highways and railroad routes. Colorado’s most expensive flood was probably the flood in the South Platte River basin south of Denver in 1965, which caused $508,000,000 worth of damage and drowned six people. The losses can be attributed to man’s failure to realize the significance of the South Platte drainage routes and flood plains. Homes, shopping centers, and many other buildings occupied—and still occupy, as of 1971—land that has been intermittently flooded for many years. The following description of this flood, by H. F. Matthai of the U. S. Geological Survey, may help to convey some warning to residents or potential residents of the South Platte valley and other river valleys in Colorado: