Laserfiche WebLink
<br /> <br /> <br />Mountain Runoff <br /> <br />Most of the runoff in the State occurs during the period <br />April to JUly in streams originating in the mountains and is <br />caused by the melting of the winter's accumulation of snow. As <br />much as three-fourths of the annual total flow of mountain streams <br />occurs in the period May to JUly, the streams usually reach their <br />peak flows in Mayor June. The seasonal distribution of flow in <br />a typical mountain stream is shown in figure 4. <br /> <br />The flow in mountain streams after the spring runoff ceases <br />is due largely to the discharge of ground water. Melting of high- <br />altitude snow or glaciers in some cases contributes a small amount <br />as late as september and rainstorms may cause increases in surface <br />runoff for short periods. From 10 to 20 percent of the annual <br />runoff of mountain streams occurs in the annual low-flow period <br />from October to March. <br /> <br />The range in annual runoff has been from about 40 to 200 <br />percent of the mean at 5 gaging stations on mountain streams having <br />records of natural flow dating back about 50 years. HoWever, in <br />more than two-thirds of the years, the annual figure was wi thin <br />20 to 30 percent of the mean. yearly variations of flow tend to <br />be similar in large parts of the mountainous area of the State. <br />Minimum or near-minimum annual flows of record occurred through- <br />out the mountains in the drought years 1934 and 1954. <br /> <br />Mountain streams have their highest concentration of suspended <br />sediments and their lowest concentration of dissolved solids during <br />the spring7 however, with few exceptions, suspended sediment loads <br />and dissolved mineral .content of the streams above 7,000 feet are <br />very low. <br /> <br />Nonmountaip Runoff <br /> <br />Streamflow originating in most of the nonmountainous areas <br />of Colorado is ephemeral. In these areas, snowmelt sometimes <br />sustains flow for a short period in early spring, but thereafter <br />runoff occurs only in direct response to heavy rains. Average <br />annual runoff ranges from practically zero to 1.0 inch, and large <br />areas on the plains east of the Rocky Mountains and in the San <br />Luis valley produce less than 0.25 inch. Yearly variations in <br />runoff are generally greater than in the mountain areas. Ex- <br />ceptions to this generalization are several streams in the Kansas <br />River basin, fed by ground water from the Ogallala formation, that <br />have quite uniform flows and the average annual runoff is as high <br />as 3 inches. <br /> <br />Water in nonmountain streams has a higher mineral sediment <br />content than that in mountain streams. Streams that rise in the <br />mountains naturally change as they traverse the lower valleys <br />and plains, unit runoff decreases, chemical quality deteriorates, <br />and sediment concentrations increase. <br /> <br />11 <br />