I must admit after visiting numerous sites within the flood zone, print images fall short of illustrating the powerful impacts the torrential rains had on the local landscape. Speaking with a number of curious individuals as CPW crews completed initial surveys of the Big Thompson, St. Vrain, and Boulder Creek drainages many questioned how in modern times such an event could have even occurred, the status of our public fisheries, and what might occur over the coming months. With respect to those personally impacted a fishery assessment is certainly a minuscule appointment however there are lessons which can be learned from the flooding.
Dams which partially tame many of Colorado’s wild rivers were built for a variety of reasons which cascade in importance. Flood mitigation, water storage, hydro-electric power generation and recreational use were the typical rationale for construction in order of descending importance. Olympus Dam (Big Thompson River), Buttonrock Dam (North Saint Vrain River), Barker Dam (Middle Boulder Creek), and Gross Dam (South Boulder Creek) respectively held back significant portions of the surging flood waters while diversions soaked up additional waters filling reservoirs in a matter of days instead of months. Water diversions from west slope sources were quickly curtailed as the rain accumulated but in the end Mother Nature proved more powerful as virtually every major of river drainage along the Northern Front Range toppled its banks and spread out over the flood plain – the low lying area along the river.
During the peak of the flood, rivers inundated the flood plain with sediments, tree snags, and a variety of rock materials. Areas which held their own in the midst to the rampage were densely vegetated with mature tree stands. It was along these habitats trout and other fish species gained refuge as the brunt of flood water raged within a primary channel. As a result I do not expect entire populations of trout to be wiped clean in any of the flooded rivers however fall surveys will help prioritize what sections will need the greatest attention.
As the waters recede the flood plain will not appear aesthetically pleasing however the nutrient rich fine sediments lined with cobble provides bed material on which vegetation will rapidly grow and actually strengthen the ability of the river to absorb future flood events. The vegetation will promote extensive insect production and willow growth utilized by a variety of wildlife including moose and elk.
Perhaps the most interesting natural feature, particularly obvious on sections of the St. Vrain, was the formation of a multi-stage river channel visible to even a novice angler or river user. The shape of the new channel resembles an inverted Incan pyramid and is better suited to accommodate extreme spring runoffs.
Directly along the bank, root wads and larger timber was jammed into the bank entrenched in fine sediment. As new vegetation sprouts over the debris fabric the banks will become stronger than any human engineered material while simultaneously providing in-stream habitats in abundance the likes of which never been measured. While leaving such materials in the river may seem counter intuitive, knowledge gained from previous events found riverine fisheries return in better shape and have a superior ability to absorb the forces created by future flood events. Provided engineers and scientists can strike the right balance among roadways, flood plains, and a multi-stage river channel life along Colorado’s wild rivers will emerge in better shape for users and inhabitants.
Ben Swigle is an aquatic biologist with Colorado Parks and Wildlife stationed out of the Fort Collins office. Ben holds a BS in biology from the University of Notre Dame and a MS in fisheries science from the University of Nebraska-Lincoln. After working 2 years as a fishery biologist with the USGS in Oregon, he joined Parks and Wildlife in 2005. Ben specifically manages lakes, reservoirs, and streams within the Big Thompson, St. Vrain, and Boulder Creek watersheds.