Trout Survival After the Floods

When you see a trout stream transformed into a frightening, raging torrent—as the Williams River was—you have to wonder how a trout could survive such power. photo by Len Emery

My home state of Vermont was recently ravaged by flooding from the rains of Tropical Storm Irene. Tiny creeks became rushing torrents, midsize rivers hit record levels, and the state’s largest rivers flowed over parts of their floodplains that rarely see water. The devastating impacts of this flooding on the residents of the Green Mountain State have been widely broadcast, and recovery will take a great deal of time. I wondered about the impacts of this flooding on the local fisheries, so I began to research what happens to fish during floods.

Eminently Adaptable
Fish are incredibly adaptable animals. Stream-dwelling trout, for example, live in an environment that has a tendency to change a fair amount. Spring flooding is part of the normal cycle for these and other animals, and they have learned to make it through this annual event for millennia. A flash-flood event is somewhat similar, albeit far more abrupt.

As humans, we see the high flood water overwhelming a river’s flood plain and washing away almost anything in its path. We have to remember that the extremely fast and destructive flow that we see on the surface is only part of the story. The fastest flow on a river follows the thalweg, an imaginary line that describes the swiftest part of a river. As anglers, we probably have a pretty good idea of where that is on our local streams. Now think of it in three dimensions: the fastest current will be under the thalweg and about a third of the depth down from the surface. Closer to the bottom will have a slower flow. This holds true during flood events. The overall velocity of the water’s flow is up, but there are still areas along the river bottom that allow fish to stay safe.

A before-and-after-Irene comparison of Vermont’s Route 4 near Mendon. The power of the flood can be measured by how many full-grown trees were washed away. photo via facebook

I spoke to Shawn Good, a Vermont Fish and Wildlife biologist, about what happens to fish during these floods. Shawn knows from firsthand experience—he did his graduate work studying Atlantic salmon in the tributaries of the Saguenay River in Quebec during the mid 1990s. Among the things he investigated were the movements of fry and fingerlings, so he had tagged quite a few in his research areas. Good was nearing the completion of his graduate field work in July 1996 when severe flooding hit the region. The area received the amount of rain that it would normally have for the entire month in just two days. Needless to say, Shawn was very concerned about completing his field work.

After the floods receded, he went back to his research areas and electrofished the river to find his marked individuals. Expectations were low when the work began but that changed quickly. Good found 95% of his research subjects were still alive and in the same location. Not only did this discovery amaze Shawn, but his findings were integrated into his thesis.

Similar results have been found in brook trout populations as well. A very high percentage of small stream brook trout only live about a year. One number I have heard often in regard to brook trout is 90% annual mortality. This mortality comes from predation, winter kill, and natural events such as spring flooding. This is a very high mortality rate for a trout species, but it also shows the resilience of brookies. Numbers rebound very quickly. In a sense, brook trout are the rats of the trout world, able to repopulate very quickly. (A note here: brook trout are losing ground throughout much of their native range in the United States, but that is primarily due to habitat degradation and loss. The resilience of a species isn’t a factor if they don’t have the proper place to live.)

Faced with this kind of damage, most homeowners won’t think twice about using heavy machinery to alter the streambed, now that restrictions have been lifted. photo by Tim Bronson

When I talked to Shawn, I also asked him about warmwater species and how they fared. I had suspected that they would do better than the salmonids. I was wrong. He told me that he felt that the warmwater species were not as well adapted to the extreme high flows, and that the mud- and silt-laden water could be much more problematic for them. One species both Good and I have an interest in are northern pike. He and I both thought that they would probably fare pretty well because they will utilize the same habitats that they do in the spring time—flooded fields and forested swamps. It could actually be a bit of a feeding frenzy for them.

Another concern we spoke about was contamination from fuel oil, kerosene, and other chemicals in the water. Checking out my local river a couple of days after the flooding, I noticed a very strong odor of kerosene near the river and a sheen on the water’s surface. I called the state Hazardous Materials Cleanup hotline and spoke to a member of the state rivers team about it. He said that they were dealing with a lot of similar situations. They were responding to every one of these problems as quickly as they could, but it was an uphill battle with the sheer number of reports they were receiving. On top of fuel tanks being directly dumped by the flooding, there are also concerns of people pumping out flooded basements which removes both the water and any fuel that may have leaked. Vermont has separator trucks that are available to help people pump their basements and prevent that fuel from getting into local rivers and streams.

The Hand of Man
Perhaps the biggest threat currently is from all the work that people are doing to recover from the storm. Many states have decided to forgo permitting for in-stream projects in order to allow people to move rivers back to where they once were and to help rebuild roads. This could actually cause the greatest damage to fisheries from these storms. There is an erroneous opinion that many folks share that dredging a river, removing obstacles, and straightening it will help to make flooding less dangerous in the future. Nothing could be further from the truth. Dredging and straightening rivers simply moves the water faster downstream, causing more intense flooding. Removal of in-stream materials, such as boulders and logs, also intensifies downstream flooding and removes critical habitat.

Low-land rivers that jumped their banks, such as the Battenkill, offered fish places to get out of the current, which should help survival rates—as long as the fish were able to get back to the main river when the water receded. photo by Tim Bronson

An unfortunate example of a township that has taken things too far already is East Middlebury, Vermont, through which flows the Middlebury River. There were several homes and businesses impacted by the flooding, so the town used the easing of restrictions to shore up the riverbanks. They put four excavators into the river and started to remove all boulders and other in-stream habitat, dredge the river to shore up the banks, and channelize the river to allow faster passage of the water. I can say from personal experience that the stretch that they have destroyed (yes, I say “destroyed,” and I mean “destroyed”) was excellent habitat for trout.

Vermont Fish and Wildlife Commissioner Patrick Berry was apprised of the situation and acted swiftly. He asked Shawn Good to make an assessment of the damages. An excerpt from Good’s report describes the damage already caused: “The fish habitat value and biological integrity of this reach of river has been significantly impacted. The stream channel and bottom is now mostly uniform across its width, with very little heterogeneity of bottom structure, and almost zero habitat complexity. Riffles, runs, and pools have been converted to one big, long chute, and the bank armoring has completely eliminated any bank-related cover or structure, such as undercuts. Additionally, it appears that all large woody debris, an important component of a healthy stream environment, has been removed from the river channel.”

This kind of human activity will have much greater long-term effects on fish populations than the flooding will. In many cases, this flooding has improved some degraded habitats. Some previously silted areas are now cleaned, pools have been scoured deeper, and much needed woody debris has been deposited into rivers. Dredging and channelizing has proven itself time and again to be an overly destructive method of dealing with flooding. The cost of repairing damages caused by doing this work can and does exceed the cost of preventing it in the first place.

I walked various parts of my local river just a couple of days after the flooding. The water was still high but it was clearing up. I was trying to see what kind of impacts the flooding had on the river itself and to see if a few of my neighbors could use help from our Trout Unlimited chapter. Some of my favorite spots are dramatically different, while others are hardly changed at all. While I was doing this I noticed something that made me smile: trout were rising in several places to Isonychia hatching.

Look at that water and try to imagine a 6-inch brook trout beneath those waves. photo by Scott McEnaney

It was very nice to see some of the locals getting back to the business of life.

One thing we all have to remember about flooding is that it is part of the normal cycle of rivers. Floods like these have been changing the paths of rivers for time immemorial. The only thing that has changed is that we are now building homes near these areas. We need to learn to adapt, rather than try to change the river to suit us. If we do that, then the fish will do fine during floods. They have lived through it before and will do so again

Drew Price lives in Northern Vermont is the owner and head guide for Master Class Angling. He fishes Lake Champlain and surrounding waters targeting carp, pike, bowfin, gar, bass and other species (even the occasional trout).