The Glaciation of Grand Lake

Grand Lake

Created by:
Scott M. Smith

Unreviewed Site

Introduction Geologic Setting
The Colorado-Big Thompson Project References


Grand Lake is located within the Rocky Mountains of northern Colorado at the headwaters of the Colorado River. The lake is the largest natural body of water in the state of Colorado. The area of Grand Lake is the western entrance into Rocky Mountain National Park. Grand Lake borders the Arapaho National Forest to the west and Rocky Mountain National Park to the east. The Continental Divide surrounds the valleys of Grand Lake which meet in a central area called the Kawuneeche valley. The Colorado River begins in this valley and flows to the west of Grand Lake and south to Lake Granby. The Rocky Mountains were created during the Laramide Orogeny, beginning during the late Cretaceous and into the Tertiary. During this period, the Rocky Mountain region experienced major crustal uplift creating beautiful mountains. Erosion of the mountains from glaciers, rivers, and streams began to carve out large valleys and sharp mountain peaks that we see today. The Grand Lake area shows impressive glaciated erosion and deposition in the form of deep U-shaped valleys, steep valley walls, hanging valleys, moraines, cirques, and many other features. Large glacial lakes can be seen in this area where cirque glaciers had formed (Aber, 2006).

Grand Lake is located in the southern portion of the map. Image taken from USGS

Image taken from NASA Earth Observatory

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Geologic Setting

There were at least two pre-Bull Lake glaciations which consists of till that is strongly weathered, and almost completely eroded areas which lack the features that characterize moraines. These deposits date back to about 300,000 to 500,000 years ago. The oldest of the intermediate glaciations in the area most likely occurred about 160,000 years ago. The deposits consists of unsorted silty sand, pebbles and cobbles of gneiss, as well as a few boulders. These deposits are only about 1 to 6 feet thick and overlie an eroded gneiss bedrock. Any glacial features from these deposits have been eroded away from the more recent glaciations (McCreary, 2005).

The Bull Lake glaciations consists of two advances which is separated with a glacial recession. The early Bull Lake advance most likely began about 127,000 years ago and ended about 105,000 years ago. The late Bull Lake advance began about 100,000 years ago and may have ended about 80,000 years ago. These glacial advances began from the same cirques and progressed down the same valleys which merged with other glaciers to form large glaciers. The major moraine deposits of these two advances can be seen today (Richmond, 1974).

The youngest glaciation of the Rocky Mountains is called the Pinedale. The glacial features of the Pinedale are well preserved which contain minimal erosion and weathering. Many ponds and lakes are characteristic of the Grand Lake area as well as the moraines that help create the lake. The Pinedale glaciation dates back to about 30,000 years ago and ended about 10,000 years ago. After the Pinedale, many smaller advances of the glaciers around Grand Lake persisted during the Pleistocene and Holocene. Many moraines and till deposits of this period can be found further up the valleys (Aber).

The mountains west of the Continental Divide created the glaciers that produced the large moraines which encloses Grand Lake. These glaciers originated from the East and North Inlet which were several miles long and about 1,500 feet thick in some places. It is thought that the glaciers receeded quickly, in less than 1,000 years which caused meltwater to flow down the glacial valleys. This created many small lakes and streams that eventually filled with sand and gravel. These features can be seen in the Big Meadows and Long Meadows area north of Grand Lake.

The end moraines of the early Bull Lake glacial advance consists of brownish, silty deposits containing many pale-gray stones with sand deposits. The glaciers descended down the Colorado River valley and deposited broad, smooth end moraines. The moraine directly south of Shadow Mountain Lake is characteristic of these deposits. These moraines contain large amounts of volcanic rock that was carried down by the glacier from the Never Summer Mountains, just west of Grand Lake. The glaciers from the Never Summer Mountains merged with the glaciers from the west side of the Continental Divide which flowed down the valley of the Colorado River. The accumulation of glaciers in the valley reached about 20 miles long from La Poudre Pass to Shadow Mountain Lake. The deposits of the late Bull Lake advance overlie the weathered deposits of the early Bull Lake advance. These deposits also contain pale brown silty sand with stones of many sizes and shapes. Some of the rocks of these moraines show glacial striations. Boulders are usually not found with these moraines.

The end moraines of the Pinedale formed islands across the southern portion of Shadow Mountain Lake. These deposits are well preserved and contain large unweathered boulders (Richmond, 1974). Many small lakes were filled in with sand, silt, and gravel deposits from the Pinedale, which created the meadows and bogs that are visible today.

Few if any glaciers remain today, however, the few glaciers that did survive can be seen in the north facing cirques where the snow accumulation is sheltered from the sun. Most of these glaciers are not progressing forward, and the formation of new ice is barely able to replace the annual melt loss. Most of the cirque basins in this area contain small ponds or lakes where the glaciers had began. The large glacial valleys are evidence of erosive power that these glaciers produce (Richmond, 1974).

Image taken from USGS

Image taken from USGS

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The Colorado-Big Thompson Project

The natural flow into Grand Lake comes from the North and East Inlet streams bringing snowmelt from the watersheds extending to the Continental Divide. The outlet from Grand Lake was through a break in the western moraine which originally formed the headwaters of the Colorado River. Before the construction of the Colorado-Big Thompson project, the area now filled by Shadow Mountain Lake was a flat valley where the Colorado River meandered. The flow of Grand Lake merged with flow from the North Fork of the Colorado River from the Kawuneeche Valley.

Located south of Grand Lake, Granby reservoir was constructed to receive runoff from a much larger watershed extending farther south. A pumping plant was created on the north shore of Lake Granby to pump water into Shadow Mountain Lake. Water is forced backwards into Grand Lake through a channel connecting Grand Lake and Shadow Mountain Lake. A tunnel was constructed as part of the Colorado-Big Thompson project which allows water to flow from Grand Lake underneath the Continental Divide to Horsetooth and Carter reservoirs as well as many other bodies of water. This project was created to alleviate water supply problems for the eastern slope of the front range mountains, but many harmful problems are occurring as a result of the project. Shadow Mountain Lake has been known to contain weeds, silt, and algal toxins, which are eventually pumped into Grand Lake, contaminating the preserved glacial lake (Stahl, 2004). This could cause major drinking water problems for the eastern slope towns.

Image taken from NCWCD

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Colorado Geological Survey
Northern Colorado Water Conservancy District
Rocky Mountain National Park

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Aber, James, 2006. Regional Glaciation of the Southern Rocky Mountains.

Batty, Steve. Photograph of Grand Lake.

Colorado State Map.

McCreary, Jeremy, 2005. A tribute to Colorados Physical Past and Present. Colorado Geology Photojournals. Last Modified January 3, 2005.

Ogden, Tweto, 1979. USGS geologic map of Colorado. 1:500,000 scale. Northwest Section. Go to this link for geologic explanation

Richmond, Gerald M., 1974. A geologic history of the mountains and of the ice age in Rocky Mountain National Park, Raising the roof of the Rockies. Rocky Mountain Nature Association, Inc. and the U.S. Department of the Interior. Library of Congress Catalog No. 73-93275.

Stahl, John. 2004. Grand Lake Water Quality and Colorado-Big Thompson Project, Project Background. Greater Grand Lake Shoreline Association.

USGS, U.S. Department of the Interior, 2006. Andrews Glacier. August 2, 1916. Plate 5-A, U.S. Geological Survey Bulletin 730-A.

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