NORTH ST. VRAIN CREEK AND
LITTLE THOMPSON RIVER RESTORATION

This river restoration project was designed and built around restoring river processes, not just river form. The features that were built are intended to encourage long-term system stability by providing a framework within which natural sediment movement and ecosystem processes can dominate. The river, and low-flow channel in particular, is expected to move and adjust; the river system is expected to recruit woody vegetation and woody debris; and the bankfull channel is expected to scour and form new deposits as it adjusts to varying flow and sediment loading cycles in the coming years and decades. Evidence of these adjustments and observations of change will deem this project a success, not a failure. A monitoring program to record and measure these changes is recommended. In order to maintain and capitalize on improvements made through this river restoration effort, important future activities to be conducted at the US 36 project sites include this long-term monitoring as well as active riparian revegetation.

The US 36 river restoration project had several constraints that influenced the recommended design, particularly the encroachment of the roadway embankment on the river channel corridor. Implementation of the river restoration project was influenced by an accelerated timeline, availability of construction equipment, and transport of needed construction materials. However, useful lessons were learned throughout the process that can benefit future flood recovery efforts and other restoration projects that involve a shared roadway/river corridor.

Point bars and alternating bars were constructed at most sites to reduce the width-to-depth ratio of the low-flow channel and to encourage natural sediment transport processes though the reach. These features increase floodplain capacity while adding channel complexity, and were constructed mainly using native cobbles, large buried rocks, and large woody debris. Point bars are features on the inside of sweeping bends, and alternating bars are present on both sides of the river, alternating between left bank and right bank, providing low low channel sinuosity when the overall river valley is straight.

Floodplain benches were incorporated in order to maintain a bankfull channel while allowing water during higher flow conditions to access the floodplain and dissipate river energy. These benches exist within the bankfull width of the channel, at an intermediate elevation between the channel bottom and the topographic floodplain or terraces.

Large woody debris was utilized in several areas for a variety of purposes, including floodplain roughness, bank stabilization, energy dissipation, and habitat complexity. Root wads were installed facing upstream on the outside of meander bends in order to dissipate energy and create micro-habitats (Figure 5). Large wood was placed strategically at an upstream, downstream, or perpendicular angle to check the direction and speed of the creek’s flow. These features were anchored utilizing large blast rock as ballast.

Boulder clusters and habitat boulders were placed throughout the channel to create habitat for fish and other aquatic species. Secondary benefits include increased channel roughness to slow water velocities and reduce sheer stress on the channel bed and  banks (including the roadway embankment). These boulders may also create recreational features.

Grade control and drop structures were built to set the elevation of the streambed in order to reduce the likelihood of severe scour, as well as to aid in the maintenance of riffle and pool bed features. The pools created by these structures provide cover for fish and other aquatic organisms. Water quality at drop structures is enhanced through aeration, and stream bank erosion is typically reduced as energy is dissipated in the vertical instead of horizontal direction.

Riffle-pool sequences were constructed throughout the project reach to provide a heterogeneous physical environment that can be used by a diversity of organisms. They provide a refuge from high velocity waters and extreme temperatures, and promote habitat complexity by offering diverse areas of cover, food, as well as spawning and rearing areas.

Backwater pools were created to increase aquatic and riparian habitat diversity and provide an area of shallow, slower-moving water for aquatic species, particularly fry and juvenile trout.