The Grand Ditch: What a Century-Old Water Diversion Does to the Kawuneeche Valley

Built over 100 years ago, the Grand Ditch carries water to the dry east side of the Continental Divide. NPS-RMNP.

The question

The Grand Ditch is a century-old water diversion in the northwest corner of Rocky Mountain National Park. Fifteen miles long, it drains about five percent of the park, catching eleven headwater tributaries of the Colorado River each year between May and September. Park managers worried about what the diversion does to the streamside and wetland communities of the Kawuneeche Valley below.

The study

From 1996, Colorado State University researchers David Cooper, Lee MacDonald, Rod Chimner and Scott Woods compared plant communities below the ditch with similar ones elsewhere. They measured surface water flow and groundwater levels across the valley, monitored gases given off by peatland soils, and estimated how much sediment was building up in the Colorado River.

Peatlands are ecosystems with organic, peat soils. Because they are waterlogged all year, there is little oxygen, dead plants decay slowly, and peat builds up; some plants grow nowhere else.

Peatlands are home to unusual plants such as this queen's crown. NPS-RMNP.

The results

Depending on the snowpack, the ditch diverts 20 to 40 percent of the runoff from the Never Summer Mountains, an average of 29%. It does not dry up the tributaries entirely, but the water table, the factor that matters most for healthy water-loving plant communities, was significantly lowered across much of the valley floor by reduced flow in the Colorado River.

The scientists found three kinds of harm:

1. Fewer floods, by as much as 50 percent, so less bare, moist ground forms where new willows and other streamside trees and shrubs can take root.
2. Weeds: lower flows may have let exotic plants suited to drier ground gain a foothold in wet areas.
3. Shrinking peatlands: when snowmelt is diverted, peatland water levels fall by up to 20 inches (50 cm), letting oxygen-using organisms break the peat down quickly. Under the current water regime, many of these peatlands are likely to disappear.

The damage to peatlands could probably be eased by releasing small amounts of water from the ditch into particular west-slope areas in late summer, but how much, and when, needs more study first.

This summary rests on reports available when it was written (September 2004, updated January 2008) and is not a statement of park policy or a definitive account of the research. More on the park's research: nps.gov/romo.

Sources

• Judy Visty, "Effects of the Grand Ditch," National Park Service, Rocky Mountain National Park. https://www.nps.gov/articles/effects-of-the-grand-ditch.htm