2013 Front Range Flooding: An Ecological Perspective

by Terri Cook
Thursday, January 2, 2014

Predatory stonefly populations may be affected by algal losses in mountain streams. James McCutchan Jr.

Boulder's open spaces host a number of bald eagles and herons that prefer large, old cottonwood trees for nesting. Many of these trees were knocked down by the floods. Managers are hoping the Front Range drainages will experience a regeneration of these native trees following the September floods. Rolf Reiser

The 2013 Colorado floods may have been a record-setting event in human terms, but scientists and resource managers emphasize that what happened along the Front Range was a natural occurrence. “This was a natural event, and the system and all the species in it are equipped to cope with such events,” says Deborah Brosnan, an environmental and policy scientist at the University of California at Davis who works on the ecological and environmental effects of natural hazards, and who assessed the ecological effects of these floods. “From nature’s perspective, this was not noteworthy,” she says.

Floods are common and “a normal, and very necessary, part of what a stream does,” says Marianne Giolitto, wetland and riparian ecologist for the City of Boulder Open Space and Mountain Parks (OSMP). Smaller floods used to occur more frequently along the Front Range, but the flows in most Front Range streams have been attenuated by human changes to the landscape, she says. Dams, irrigation ditches and stream channelization (straightening and widening of the channel) have all had significant impacts.

“Collectively, these things have changed the flow rate and the flood events that historically would have happened here,” adds OSMP interpretive naturalist Lynne Sullivan. What once would have been a large spring pulse of water is now intercepted and gradually dispersed over the landscape. “The water continues to trickle into the creek for a much longer period of time, creating flows downstream later in the season that would not have been there historically,” Sullivan says.

The result of all these changes, says Heather Swanson, an OSMP wildlife ecologist, is that these ecosystems now look very different than they would with frequent, small flooding events still occurring. “It now takes a big event to bring an ecosystem-changing disturbance back.”

While the physical damage to human infrastructure from such a large and widespread event is easily quantified, assessing the ecological effects is much more difficult. One reason is that, despite the term being widely used, there is no agreed-upon definition of an ecological disturbance, in part because such disturbances play an important role in structuring stream ecosystems, says James McCutchan Jr., associate director of the Center for Limnology at the University of Colorado at Boulder. “Many ecologists define a disturbance as ‘a discrete and unpredictable event that disrupts biological community structure or ecosystem function,’ but stream ecologists have debated for years about how to define disturbance in the context of stream ecology,” he says.

There is also the question of the appropriate time frame to evaluate. As with any large disturbance of a landscape, there’s a big difference between immediate and long-term impacts, Swanson says. Managers and researchers are also limited by what they can directly observe, which can skew assessments of hard-to-see impacts such as wildlife mortality.

Nonetheless, scientists and resource managers were out daily after the September floods, trying to assess landscape and ecological changes. I went out with several of them to see the damage firsthand. Though it will take months to years to grasp the long-term effects, preliminary observations of the immediate impacts on the local flora and fauna will help resource managers figure out what they need to assess, how best to apply their limited funding, and how to direct recovery projects staffed by volunteers.

The Impacts on Wildlife

In the long run, the flooding is likely to benefit many native wildlife species. Ecologists consider periodic scouring floods essential for maintaining a healthy ecosystem along the vital Front Range drainages. Like after a large wildfire, which the area has also recently experienced, the ecosystem has the ability over the long term to rebound “beyond what it was before,” Swanson says, “so many species actually benefit following a disturbance.” In the short run, however, the flooding caused some mortality.

“One species that is easy to observe is prairie dogs,” Swanson says. “We saw several colonies along the creeks that had water flowing right over them.” Prairie dogs can swim, but Swanson doesn’t know whether the affected individuals moved to higher ground or drowned; all she can say is that these colonies are no longer active. In the long run, Swanson says she is not too concerned about their population numbers. “There was some immediate mortality, but only a small number of colonies were completely wiped out, and we expect the prairie dogs will quickly repopulate,” she says.

Burrowing animals, including rodents and snakes, probably suffered the greatest losses, being the most likely to drown or get swept away by the floodwaters, Swanson notes. Anecdotal reports of dozens of dead snakes found on a local highway add credence to Swanson’s suspicions.

Many of the riparian, or riverbank, species, which are naturally adapted to flooding, will likely be the biggest long-term beneficiaries, Swanson says. One exception may be the Preble’s meadow jumping mouse, which is federally designated as a threatened species, whose critical habitat includes riparian areas along several of Boulder’s open space drainages. Although this mouse is a riparian species adapted to flooding, Swanson says that due to the unusual timing of the floods, which coincided with the beginning of the hibernation season, typical hibernation sites may have been lost. Additionally, some of the older individuals may have already entered hibernation in burrows located close to the creeks. “If they were underground, they probably drowned,” Swanson says.

In addition, because the storm affected all of the drainages along the Colorado Front Range north of Colorado Springs, the isolated and scattered populations of Preble’s mice could have trouble bouncing back. “If something happens to one population, it could have a pretty big impact on the species,” she says.

Open space managers hope to revise Preble’s population estimates next year to compare with previous censuses to determine whether the tenuous populations took a hit in September. Fortunately, Swanson says, most of the rest of the Front Range wildlife — including everything from tassel-eared squirrels to black bears and bobcats — are far more common and have “a much better ability to absorb that short-term negative impact and capitalize on the long-term gains.” Nonetheless, Swanson says she and her team will continue to monitor other species, including northern leopard frogs and riparian birds, and will collaborate with state biologists to monitor fish populations.

Birds, Bats and Trees

Boulder’s open space lands host a number of nesting bald eagles and herons that prefer the large, old cottonwood trees, whose life spans are naturally limited. So an ongoing challenge that OSMP managers have been facing is whether there will be a healthy, new generation of middle-age cottonwoods to replace the old ones as they topple. “In a lot of our riparian areas, the answer is no. We don’t have those younger age classes coming up,” Swanson says. OSMP managers say they hope that the “disturbed” drainages will experience a regeneration of cottonwood and willow trees: Because these native plants need direct sunlight, exposed mineral soils and access to the water table for their seeds to germinate, they thrive on recently scoured gravel bars.

In 2011, Boulder’s OSMP division transplanted willows and built structures along some of the local creeks to increase the trees' chances of regeneration, but where nature is allowed to take its course, like it did in September, “it does a lot better than we do,” Giolitto says.

Although bats don’t live directly within the drainages, their populations along the Front Range were also likely affected by the torrential rains, although as with many species, the cumulative effects on their populations are difficult to predict — and are a concern for managers.

Maternity colonies of species — such as the fringed myotis, little brown myotis, small-footed myotis, and the big brown bat — located in rock crevices, under rocks on scree slopes, or under tree stumps are particularly vulnerable, says Rick Adams, a chiropterologist at the University of Northern Colorado who has studied bats in the region since 1984. By the time the heavy rainfall occurred, many of these had likely begun to disperse to hibernation sites. “However,” he says, “the local migration from lower-elevation summer roosts to winter hibernacula located in underground caverns high in the mountains is a bit tenuous, especially for the young born that summer because they don’t know the area yet and tend to end up in more precarious situations.”

Long-distance migratory tree bats also found along the Front Range are much more susceptible to weather because they are directly exposed to the elements as they roost in the open on branches or just under parted bark on the trunk, Adams says. September is a prime migratory season for these species, so it is possible that many individuals were killed by the severe rains. “Because bats only have one [offspring] per year and most of these do not survive their first year, it takes decades for populations to bounce back from a dramatic decline,” he says.

In addition, bats are dependent on insects such as mosquitoes, mayflies and stoneflies, many of which spend the larval period of their lives in streams and ponds, and no one knows yet how the extensive flooding may have affected these populations, Adams says. Some of the water sources where Adams has been tracking bats for more than 15 years were destroyed, which may also have a negative effect “by removing small stream waterholes near maternity roosts that lactating females desperately need in our hot, dry summer climate.” On the other hand, he says, scouring in the drainages removed much of the low-growing vegetation, which improved flight paths and created more areas where summer water may pool, thereby increasing water availability near maternity roosts in some areas.

The bottom line, he says, is that no one will understand the impacts on bats until this spring, when post-flood fieldwork will begin and scientists better understand what landscape changes have occurred. They will be monitoring populations that they have tracked since 1995. In addition, they’ll rely on their volunteer bat monitoring program, which since 1996 has used citizen scientists to report on roost sites and water sources and to monitor bat activity. The volunteer monitoring in summer 2014 will be instrumental in understanding the flood’s effects on local bat populations, Adams says.

Effects on Streams

Severe floods can affect not only a stream’s channel but also its riparian zone, riparian predators and transfers of organic matter between the riparian zone and the stream channel. The effects of riparian disturbance may be particularly long lasting, according to McCutchan.

The specific ecological effects of the heavy rain and flooding along Front Range streams can vary depending on the distance from the stream headwaters. For instance, flood damage will be very different in high-gradient, montane zones versus the flatter plains. Montane areas would typically see movement of large boulders and debris like trees, whereas the plains will experience much more widespread flooding and sediment deposition as floodwaters spread out across the floodplain. Likewise, the addition, transport and deposition of significant amounts of sediment can also affect the metabolic processes that occur in a stream, including oxygen metabolism (photosynthesis and respiration) and other processes such as denitrification, which reduces nitrate and produces nitrogen gas.

One of the biggest ecological effects of flooding concerns algae. As the base of the food chain, algae are important for the whole ecosystem, so McCutchan and others have been out in the field measuring the amount of algae that remained after the flood and their rate of recovery.

“Floods like the one in September move significant amounts of material in the river’s bed, which can remove the attached algae, the most important food source for many invertebrates, and temporarily change the food supply for stream food webs,” McCutchan says. “Mobilization of the bed can also cause high mortality of aquatic insects.”

Near the headwaters of alpine streams at high elevations in Rocky Mountain National Park, McCutchan has observed that the flooding removed some but not all of the attached algae. At lower elevations where the flooding was more intense, he saw that the floodwaters had removed virtually all of the attached algae. These findings are consistent with his impression that, in general, the most extreme ecological effects of the flooding appear to have been in middle- and low-elevation streams, including many reaches in the North and South St. Vrain rivers, which merge near the heavily flooded foothill town of Lyons.

Farther downstream, as the streams meander across the Great Plains, the ecological effects, and the recovery from such disturbances, differ. The river water is typically warmer because more sunlight reaches the wider channels. In mountain streams, even during the summer months, it may take several months for the attached algae populations to recover because of cooler water temperatures and lack of sun reaching the stream channel, McCutchan says. Recovery of algae populations in plains streams can happen much faster. “Once flows drop and the sediment load is low enough for light to reach the bottom, it might take only a few weeks for the post-flood photosynthetic rates to come back,” he says. So McCutchan and his colleagues will be watching to see how quickly algal populations recover after the flood.

The other challenge to the algae is landslides; increased transport of fine sediments may have local ecological effects for many years. “Fine sediments can affect attached algae, and by filling spaces between larger grains, fine sediments also can decrease oxygen concentrations within the stream sediments,” he says.

But sufficient oxygen concentrations are important both for invertebrates and trout and other fish species. Because fish have longer life cycles compared to many invertebrates, their recovery from ecosystem disturbance can lag far behind the recovery of algae and invertebrates. This lag can be compounded if flood damage extends to nearby reaches of the stream that would normally serve as source areas for recolonization. Over time, the balance between erosion and transport of sediment will come back into equilibrium, but this may be a change that in some places lasts for years, according to McCutchan.

Keeping It All in Perspective

While grappling with the enormous changes wrought by the floods on the Front Range landscape, flora and fauna, the region’s scientists, resource managers and citizens are using these events to reflect upon the role that large storms have played in the long-term evolution of the Front Range landscape, and the place that humans have within this context.

“Humans and nature don’t always work on the same timescale. Nature will recover and certain species will benefit from this event, but humans don’t have that luxury. We have to pay our mortgage at the end of the month,” Brosnan says.

Dave Sutherland, an OSMP interpretive naturalist who has led many post-flood interpretive hikes, likens the experience to the phases of grieving: He tries to enhance the visitors' emotional recovery by shifting the group from initial feelings of denial, anger and loss to an acceptance phase “where they can see the change as a potentially positive thing from an ecological and geological point of view.”

While he and other interpreters are careful to make a strong distinction between human losses and ecosystem gains, Sutherland tries to help participants in his programs realize that, from what he calls a natural system dimension, the change wrought by the floods is actually a beneficial thing.

These naturalists and resource managers are still developing specific plans for monitoring the sensitive species and ecosystems that were disturbed by the floods, but one thing is for sure, they say: There’s a lot of work to be done.

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