The South Platte River north of Denver is the last major commercial gravel resource in the Denver metropolitan area. Most of the deposits upstream in the South Platte, Clear Creek, and Bear Creek have been mined or precluded from mining by urban development. North of Denver, gravel mining has steadily moved downstream since the early 1970's, and now may be approaching the downstream limit of commercial viability.

When the deposits north of Denver are exhausted or preempted by other land use, aggregate for the Denver area will by necessity come from stone quarries in the mountains or from gravel deposits in valleys to the north, such as the St. Vrain River.  Increased transportation costs will significantly increase the cost of sand and gravel in the Denver metropolitan area.

Figure 1.  Map of the Project Demonstration area with close-up of the South Platte River.

The PROJECT DEMONSTRATION AREA map shows the drainage basin of the South Platte River and its tributaries in the project demonstration area from Denver to Ft. Collins and Greeley. Named streams are in valleys with known or probable commercial deposits of gravel. All other streams lack commercial gravel deposits, although they may contain local occurrences. The SOUTH PLATTE STUDY AREA map shows the area north of Denver where the USGS has conducted detailed studies of commercial gravel deposits.

The section above shows sand, gravel, and other deposits of the South Platte valley in the vicinity of the Howe pit. The valley here actually consists of three valleys. The largest (and oldest) valley was filled by the high terrace deposits (called "Broadway") east of the Howe pit. Gravel in the old valley is covered by a thick deposit of windblown dust. Next in size and age is a remnant of a younger terrace (called "Piney Creek"), dating from the last glaciation. The young terrace forms a small step at Brighton Road, between the level of the high terrace and the modern valley surface.

Figure 3.  Photograph showing a commonly used method to sample gravel

The photo on the left shows a commonly used method to sample gravel. A vertical trench is first cut in the gravel face. Gravel is then scraped from the trench with a hoe or shovel and collected at the bottom in a bucket or on a tarpaulin. Depending on the coarseness of the gravel, about 30-100 lbs are collected for a sample. Much of the variation in estimation of particle size comes from collecting the sample.   Figure 3.  Photograph of a sieved sample.   After a sample is collected, it is sieved. Sieves like the ones in the above photo are used to catch the proportion of gravel larger than 1 = > 3/8, and 3/16 inches. Material that passes through the 3/16 inch sieve is coarse sand or finer.

U.S. Department of the Interior, U.S. Geological Survey URL: http://rockyweb.cr.usgs.gov/frontrange/virtour/howepit.htm Contact: Carol Mladinich  mailto:csmladinich@usgs.gov Updated: 05/16/2001