Culverts as culprits:
Restoring stream continuity requires more than just dam removal
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By Ethan Nedeau
Thankfully, transportation engineers are not so sadistic. Our roadways are kept smooth. Hazards
are well marked. Detours direct traffic when necessary. If a road becomes too congested, lanes are
added, bridges are widened and new roads are built. Our society spends billions of dollars annually
to allow easy travel for millions of motorists no matter the size of their vehicle. You could even
tow your house from Michigan to Maine if you were so compelled.
The network of streams, rivers and riparian corridors are not unlike our transportation routes.
The “motorists” are comprised of a rich diversity of invertebrates, fish, amphibians, reptiles,
mammals and birds, many of which need to get from place to place by moving through the water or
along the banks. Some fish species travel hundreds of miles to reach spawning areas and they rely
on an unimpeded path. Other wildlife may only move a few hundred feet to seek favorable areas.
Whatever the scale, the persistence of populations and species relies on connectivity between
critical habitats, just as your trip to the hospital or grocery store relies on safe and effective
travel routes.
Humans have severely restricted watershed connectivity. Dams are a major culprit. Dams receive
intense scrutiny because they are large and their effects are obvious: they stop water. However,
you do not have to stop the flow of water to disconnect a stream or river. The way we design our
stream crossings when we build roads - including where we cross a stream, embankments we create,
the type of culvert or bridge we use and how a culvert or bridge is designed and installed - will
determine whether a crossing breaks stream continuity. For fish that cannot jump, a six-inch drop
at the end of a culvert is just as insurmountable as a 50-foot dam.
Culverts far outnumber all other types of stream crossings. Although the 5,000 dams on the U.S.
side of the Gulf of Maine sound like a lot, culverts probably outnumber dams at least 10:1 on
fish-bearing streams, with tens of thousands more on intermittent streams. Culverts were designed
to be easy and inexpensive to install, but rarely did design specifications or installation
consider wildlife passage, natural flow dynamics and physical habitat of the stream. Most culverts
simply get water from one side of the road to the other. There are three major problems with
culverts and other stream crossings:
1. Undersized Crossings
Undersized crossings restrict natural stream flow, leading to higher water velocities within
and downstream of the crossing, scouring and erosion of the stream channel and banks, and sometimes
ponded water upstream of the crossing. Undersized crossings are often damaged during floods, when
too much water is forced through too small a space.
Maximum swimming velocity varies widely among different fish species. Salmon and trout can swim
very fast and maintain speeds over a great distance. Darters can accelerate quickly but only swim in
short bursts. Juvenile eels swim slowly and prefer to avoid fast currents. The ability of any given
species to swim through a culvert depends on how fast the water is moving, the length of the culvert,
and environmental conditions such as water temperature that affect swimming performance.
Imagine if you go to an airport with a moving sidewalk, have the engineer crank up the speed to
5 meters per second, and have it be 50 meters long. Only those people who can exceed the sidewalk
speed and maintain that speed long enough to reach the other side can continue on their trip
(I suggest carrying less baggage). Everyone else must continue trying, and if they do not succeed,
must go home.
2. Shallow Crossings
Shallow crossings often have water depths that are inadequate for many species to swim past.
Combined with a substrate that promotes homogenous flow - such as smooth cement or corrugated
steel - shallow crossings represent a major impediment to fish. Water passing through large box
culverts with a cement bottom is often dispersed across a large area and only centimeters deep.
Water becomes shallower during dry periods, and if the crossing is elevated at the upstream end,
may dry up altogether. This is why it is important for designers to consider the full hydrologic
range of a stream before installing a crossing, to ensure the crossing maintains enough water during
low flows.
Fish that try to swim through shallow crossings may suffer physical injury from scraping against
the bottom and flailing their bodies to maintain momentum. However, fish can swim through surprisingly
small amounts of water. There is a famous picture from Washington State of chum salmon crossing a
road because the culvert was flooded. The salmon were lined up along the roadside, and when a
passing car would create a wake, the salmon would bolt across the road. There was probably only 2
inches of water on the road and most of the fish's body was out of the water.
Here's an exercise for you to try: Locate a shallow (less than one foot) section of stream, more
than 30-40 feet long. Bind your hands behind your back and put flippers on your feet. Remain on your
belly and only use your feet to propel you (you may want to use knee pads). For an added challenge,
try it during high flows when water velocity is fastest.
3. Perched Crossings
Perched crossings are above the level of the stream bottom at the downstream end, creating a
miniature waterfall. Perched crossings usually get worse over time as the streambed erodes and a
scour pool develops below the culvert. Even drops of a few inches will pose a nearly insurmountable
challenge to many species of fish and invertebrates; individuals trying to get upstream are forced
to congregate below the culvert where they are vulnerable to overcrowding, predation and fishing.
Culverts often have more than just one problem. Undersized and perched crossings may be
insurmountable for even the strongest swimmers and highest leapers at high flows. Perched and
shallow culverts may be insurmountable at low flows because there is not enough concentration of
flow to direct fish. These culverts often mark the end of the road for species trying to get
upstream.
Here's an exercise for you to try: Install a 6-foot wall across the community bike path and
tally how many people cross it and how many are forced to turn around. Release a mean looking dog
as people approach the wall and see if the threat of predation affects their climbing ability.
Open an ice cream stand on one side of the wall and see if the lure of ice cream is enough to get
people across.
New Guidance
The Massachusetts Riverways Program has recently produced the Massachusetts Stream Crossings
Handbook that “informs local decision makers and advocates about the importance of properly designed
and maintained culverts and bridges for fish and wildlife passage.” It presents stream crossing
guidelines for new stream crossings and guidance for decisions to replace or retrofit existing
crossings. Although the guidelines may be subject to future revisions, the general goal will remain
the same: stream crossings should always maintain appropriate flow and substrate through the
crossing and not constrict a stream.
Recommendations contained in the Massachusetts Stream Crossings Handbook are applicable
throughout the Gulf of Maine region. They would fill a critical regional need because public
awareness of river restoration and the need for technical guidance is increasing. While dams
are scrutinized at a state, provincial or federal level, culverts can be addressed by local
citizens and municipalities. Such a community-based environmental initiative would have a lasting
positive influence on individual watersheds and the Gulf of Maine as a whole.
The truck driver who got wedged under the low bridge can certainly attest to the need for a large
enough opening to safely cross a barrier. We should not have to experience such startling (and life
threatening) moments to relate to challenges that wildlife contend with when trying to get past
poorly designed stream crossings. Get to know your local stream - what lives there, what habitat
each species prefers, and the extent to which each species moves. Climb down over embankments at
different seasons and imagine you are a darter that can only dart, or a wood turtle trying to reach
a distant riparian meadow, or an American eel that traveled thousands of miles to finally be stopped
by a perched rusted-out culvert. Surely, you will then understand the need to invest money in our
river corridors to keep them as clear as our transportation corridors.
Copies of the Massachusetts Stream Crossings Handbook will be available online this fall on the
Massachusetts Riverways Program Web site: www.massriverways.org.
© 2005 The Gulf of Maine Times
I WAS DRIVING into town the other day, and as I descended the hill to cross under the railroad bridge,
a partially decapitated truck had plugged the lane and workers were scrambling to unwedge it and tow
it away. It happens every so often; perhaps the drivers are on their cell phones as they speed past
the “Caution: Low Clearance” sign. My sadistic self imagined the turmoil one could cause by building
such impediments along our roadways, without warning signs, and seeing how Darwinism would sort
the “fit” from the “unfit” vehicles and drivers. It could be a great way to relieve congestion and
keep smoke-belching pickup trucks from our quaint downtown streets.
Amphibians and reptiles often travel along stream corridors to breed, feed or disperse to new
habitats. Small culverts do not afford an opportunity for these species to crawl through,
especially during high flows when the culvert is filled with fast-moving water. Frogs, salamanders,
turtles and snakes will hop, walk, or slither up embankments and cross roads and highways to get
around these barriers, exposing them to a significant mortality risk from vehicles or predators.
Hundreds of thousands of animals are killed each year trying to cross roads because their preferred
travel routes were encroached on by roads and poorly designed stream crossings.
Ethan Nedeau is a science translator and illustrator for the Gulf of Maine Council. He can be reached
at ethannedeau@comcast.net.