Flannelmouth suckers only had a 2% passage probability, making for an almost impassable barrier

A study published in October by Colorado Parks and Wildlife (CPW) researchers suggests features built at man-made whitewater parks (WWPs) are having negative impacts on fish passage.

The three fish species the study focused on were brown trout, Colorado sculpin and flannel-mouth suckers.

One of the study’s lead authors, Eric Richer, said CPW first began looking into the impacts of whitewater parks on fish passage a number of years ago after one scientist, an avid angler, was fishing near the whitewater park on the Arkansas River near Buena Vista.

Richer said the researcher noticed he was catching more fish downstream of the man-made structure than he was as he worked his way upstream through the structure.

The study, published in the peer-reviewed journal “River Research and Applications,” looked at two Colorado whitewater parks that have incorporated fish passage structures into their design; the Pumphouse Recreation Area along the Colorado River and along the Uncompahgre River near Montrose.

CPW was also aware of these two projects through the Army Corp. of Engineers permitting process and was able to design a study that modeled fish movement before and after the construction of the white water parks.

Currently there are over 40 WWPs completed or in the planning phases within Colorado, occurring in all major aquatic habitat zones, however, researchers found that although WWPs may provide economic and recreational benefits for local communities, WWPs can have unintended impacts on instream biota (animal and plant life of a particular region) as well as river functions.

The WWP at Pumphouse features a constructed boulder island that splits the river into two channels. The channel on river left of the island was designed as a play wave, and the channel on river right was intended to provide fish and boat passage.

The WWP at Montrose includes six structures, each consisting of a play wave formed by a prefabricated concrete slab with a design slope of 11% and narrow (~1 m) fishway that was separated from the WWP chute by a row of closely spaced boulders.

Each study site was surveyed in 2014, one year prior to WWP construction, using a combination of topographic and bathymetric surveying equipment. Surveys were then replicated following the completion of WWP construction in 2015. 

To study the impact of the whitewater park structures on fish passage, researchers completed detailed surveys of the channels at both sites before and after the construction of the park structures.

According to the study, the scientists then used a two-dimensional hydraulic model to look at water depths and velocities, which they compared to fish passage criteria for the three species of interest at different stages of their life cycles: juvenile, average adult and large adults.

Richer said for fish to get through a whitewater park structure it’s a relatively short distance of only a few meters, and the researchers assumed the fish would use burst or sprinting swimming capabilities to do so.

However, the study concluded that after whitewater park construction, brown trout, the strongest swimmers, maintained the highest proportion of passage probability at about 23%, followed by Colorado sculpin at about 15%. But for the Flannelmouth suckers, the whitewater parks appeared to be nearly impassable and a complete barrier, with only 2% passage probability.

In addition to using models, the researchers also captured fish at the Pumphouse site, marked the fish and moved them downstream of the whitewater park structure, then came back a year later to see how many of the marked fish had made it upstream of the structure.

Again, adult trout appeared to make it upstream past the whitewater park while fewer juvenile trout succeeded. The researchers also found there was also a higher concentration of suckers below the whitewater park structure than above it, suggesting the park could be impacting the suckers’ movement.

The full study from the CPW can be read online here.