RIVER COURSE CHANGES RESULTING FROM GRAVEL MINING IN CENTRAL TEXAS

Extensive commercial sand and gravel mining occurs in the Colorado and Brazos Rivers floodplains. In the absence of regulations requiring land reclamation, gravel pits are left open and abandoned, without levees or other protective measures. Flooding has caused the river to erode its banks and carve new paths through abandoned pits, effectively altering the river course at several locations in Travis and Colorado counties in Texas.

The geomorphic term for this occurrence is a “cutoff”, where a new and relatively short channel is formed when a stream cuts through the neck of a bend in the river. Meander cutoffs occur in many floodplains, and are a natural result of flooding. Artificial cutoffs, however, have unintended effects on people and the environment.

Possible impacts of artificial cutoffs include channel instability, erosion and deposition, effects on aquatic habitat and riparian ecology, and even property boundary changes. Property boundaries are affected when the line separating the state-owned riverbed from private property is altered. In Texas, these “gradient boundary lines” are legally defined by channel morphology – an artificial change in the shape or location of the river channel can literally enlarge or reduce a person’s property.

The environmental effects of artificial cutoffs are more subtle. Even if water quality is not affected, which is arguable, there are other possible impacts. Aquatic habitat needed to support game fish and threatened species may be degraded by changes in flow velocity, channel depth and substrate material, all of which result from artificial cutoffs. Excessive turbidity caused by suspended sediment can reduce light penetration in the water column, thus affecting aquatic plant growth. Riparian ecology, including the habitat of terrestrial plants and animals, is directly impacted when a river changes course.

This paper examines several specific sites where artificial cutoffs, at the location of active or abandoned gravel pits, have changed the path of a river. The effects of these artificial cutoffs are evaluated.