The negative impacts of lowhead barriers
and the varying effectiveness
of barrier removal
Austin Clarridge
Introduction
Habitat destruction and alteration is one of the most prevalent causes of species
extinction and endangerment worldwide (Dean et al. 2002). Most animals, including aquatic
ones, require a diverse group of habitats throughout their lifetime. It is extremely important that
fish and other species are given access to critical habitats such as feeding and spawning areas.
All over the world, this migratory freedom is being challenged by the installation of lowhead
barriers for flow control, hydropower, and many other purposes (Lucas et al. 2009). A lowhead
barrier is a barrier in a moving body of water measuring between 0.4 and 3 meters high, of which
there are currently about 2 million in the United States alone (Dean et al. 2002).
Some fish can migrate hundreds of miles to reach suitable spawning habitat (Beasely and
Hightower. 2000), and although lowhead barriers are less restricting than larger structures, they
are often built in higher numbers making them a severe hindrance to migratory species (Lucas et
al. 2009). These negative impacts of lowhead barriers are drastically decreasing the success of
many transient species, and much extinction, especially locally, is being found (Lucas et al.
2009). Two leading causes in the decrease of fitness in aquatic species due to lowhead barrier
installation are habitat fragmentation and migration hindrance, as well as drastic alteration of the
aquatic habitat characteristics. Both of these have been studied extensively in respect to larger
structures, but recently a movement has begun to gather more data regarding smaller lowhead
barriers. The results and findings of several of such studies will be explored throughout this
paper. The possible solution of dam removal will also be detailed regarding the successes and
failures. As more studies become available and results synthesized, action can be taken to
reduce the severe negative effects of lowhead barriers.
Effects on biotic factors
Species richness data is one of the easiest ways to see the effect of lowhead barriers on
fish migration. Studies show that species richness is actually higher directly downstream and
upstream than in the similar collection points in reference streams. In contrast, species loss was
greater from downstream to upstream in barrier streams versus free flowing ones. (Doddl et al.
2003). Higher species richness may seem to show that lowhead barriers are actually beneficial to
fish, but the opposite is true. Fish that attempt to travel to reach other habitats are stopped by the
barrier, causing a “stacking-up” of species that will increase the species richness on either side of
the dam (Doddl et al. 2003). One study showed an average upstream species loss of 4.1 in
streams with lowhead barriers, and only 1.5 species lost from downstream to upstream in free
flowing bodies (Doddl et al. 2003). Some tagged individuals have even been found to travel to
the barrier repeatedly, showing a desperation to travel to the habitats upstream (Lucas et al.
2009).
Many species prefer to spawn upstream of the areas they frequent due to many factors
including availability of small organisms for food, optimum flow rates, and availability of
spawning structure (Lucas et al. 2009,Beasely and Hightower. 2000). Lowhead barriers make it
difficult for spawning individuals to reach their preferred habitat. A study using tagged river
lampreys showed that only 1.8% of individuals actually spawned in an upstream location that
was home to 98% of the river’s preferred spawning habitat. The only individuals who
successfully reached the spawning grounds crossed the barrier during a period of flooding (Lucas
et al. 2009). A similar species conducted on shad and striped bass characterized the preferred
spawning habitat of each species based on water velocity as well as substrate size and type. It
was observed that in a river where travel was impeded by a lowhead barrier, a significant of
portion of individuals spawned in areas with characteristics very different from the preferred
(Beasely and Hightower. 2000). If the habitat varied from the preffered enough, this could
greatly decrease spawning success, and lead to decreased fitness or even extinction.
Restricted migration can also negatively affect populations that require a host to assist
them in travel (Dean et al. 2002). This is most well studied in endangered mussels and lampreys,
whose already low numbers are being all but helped by lowhead barrier installation. The study of
endangered mussels and their interactions with lowhead barriers is a very important topic right
here in our back yard in central Ohio. A study on several species of mussels in a river in Kansas
river. quantified these negative effects by collecting species data above and below lowhead
barriers. This study compared evenness, species richness, and overall abundance above and
below lowhead dams as well as similar sampling locations in reference streams. As compared to
the reference, barrier streams had lower values for all three parameters. This can partially be
explained by the inability of host fish to carry the mussels past the barrier. It was also observed
though, while overall abundance between the two site types was similar, evenness and species
richness were significantly different. This was due to the loss of two mussel species that are
considered sensitive to environmental change (Dean et al. 2002). Since the mussels were
blocked from optimum habitat, they were all lost in the barrier stream. This can also be
attributed to local habitat alteration.
Effects on abiotic factors
Stream habitats are constantly changing, and are very responsive to atypical conditions.
Human interactions are constantly altering the natural cycle of streams and rivers, and lowhead
barriers are just one way that is changing the dynamic river system. It has been found that
lowhead dams can affect streams for 4 km upstream of the structure (Evans et al. 2007). Though
they still allow water to pass over, they may have severe effects on the geomorphology, water
quality, flow rates, and other abiotic factors of the stream. The barriers hold back water, and
studies show that, on average, the depth and width of obstructed streams is higher both above
and below the impoundment. The barriers slow the flow of the stream and often trap sediment
upstream, resulting in a silty environment upstream, and a silt deficient environment downstream
(Doddl et al. 2003). Streams are very dependent on a form of silt transport called sediment
waves. These are “waves” of sediment that move downstream through a system and interacting
with it, sometimes gaining in size, and sometimes depositing silt (Evans et al. 2007). A study of
sandbars downstream of a lowhead dam, compared their characteristics and locations before and
after the installation of the dam. It was found that the dam caused the downstream moving
sediment wave to disperse when it reached the dam, causing a very silt rich environment
upstream, but also completely altering the morphology of the sandbars downstream. They grew
in size, became more oblong in shape, and migrated towards the banks. This further slowed the
flow below the dam, by lengthening the path of the main stream channel (Evans et al. 2007).
The slow flowing, silt rich environment upstream also became optimum habitat for invasive
species such as the common carp, which thrive in slow dirty water, and can outcompete native
species in such waters (Doddl et al. 2003). Erosion was found to be greatly reduced in streams
with lowhead barriers as well (Evans et al. 2007). This may seem beneficial, but rivers rely on
erosion to help them find the best path, and cut a suitable channel. A lowhead barrier can make
it difficult for a stream to achieve this goal. Lastly, lowhead barriers act as a catch to woody and
other types of structure moving downstream which would eventually come to rest and become
suitable living and spawning habitat for many aquatic species (Doddl et al. 2003).
Dam Removal
As more research is done on the negative effects of lowhead barriers, it is becoming
common for less important barriers to be removed. Though it seems like this would
immediately return a stream to its previous condition, this isn’t necessarily true. While some
factors recover quickly, others may take years to return to pre-dam conditions. Within one year,
it can be expected for flow rate to be returned to pre-dam conditions (Stanley et al. 2002). It was
found that after removal of a lowhead dam, the sediment wave is nearly immediately
rejuvenated, and is now able to continue downstream (Evans et al. 2007). This allows for the
stream to return to pre-dam balance of erosion and deposition, yet it will take time before this
cycle can return the size, shape, and location of the downstream sandbars to the original.
Depending on the characteristics of the stream, the amount of time to return to free flowing
characteristics is about one year (Stanley et al. 2002). Similiarly, fish and macroinvertebrate
populations began to shift towards free flow conditions immediately after the breaking of a dam;
however, it takes approximately two years until macroinvertebrate populations return to pre-dam
conditions. While species total taxa decreased post removal, implying the end of the “stacking
up” of species previously discussed, other parameters such as biomass, diversity, and fish density
remained altered from free flow conditions for beyond three years post removal (Stanley et al.
2002). It was also seen that the presence of invasive species such as carp became less common
shortly after the removal of the dam(Stanley et al. 2002). Particle size in the water and on the
bottom actually was found to increase post removal (Stanley et al. 2002). This seems to be the
opposite of what would be expected, but it can be possibly explained in two ways. First, the
presence of the barrier kept large particles from being carried downstream, and now the
increased flow allows transport of larger substrate. Second, scouring of rocks while being
thrown around by the dam causes the substrate to be smaller on average (Stanley et al. 2002).
The data collected after the removal of a lowhead barrier suggests that removal does
indeed assist in returning a stream to its original conditions. It must be understood, though, that
this process will take time and some factors may never recover entirely. Streams are dynamic
environments that change very rapidly when something such as the removal of a lowhead barrier
occurs. These rapid geomorpholical changes may make it hard for species to recolonize areas
around a dam, explaining the slow response. Also, the absence of suitable living and spawning
structure still remains downstream, even after the breach. It could take a long time until the area
becomes optimum for total recolonization. Most studies done on dam removal are short term
and fail to fully describe the return of the system to pre-barrier conditions. While it does appear
that dam removal can be supported as an effective conservation practice, long term studies need
to be done in order to fully understand to what extent they are effective.
Conclusion
Though lowhead barriers may seem harmless compared to larger structures and
alterations, their high numbers, hindrance to migration, and subtle yet important habitat
alterations make them a dangerous addition to river systems (Lucas et al. 2009). It is important
that the possible negative impacts of a lowhead barrier installation be thoroughly investigated
prior to beginning the project. As studies have shown, the habitat and populations effected by
the barrier will be damaged severely. While at times lowhead barriers may be necessary, it is
crucial that the number of them, primarily within the same system, be kept to a minimum. There
is a need for long term effects studies of barrier installation and removal, as well as insight into
effect minimizing strategies. One such strategy that is being utilized now is fish passage ways,
that allow fish and other migratory aquatic life an alternate route through or around the dam
(Lucas et al. 2009). Similarly, ways of reversing negative effects of lowhead barriers, and
assisting in the natural reversing cycle following barrier removal need to be explored. The
world’s rivers and streams provide countless benefits to human life from scenic, to recreational,
to economic factors. People are constantly interacting with them in many different ways. While
the benefits of lowhead barriers are extensive, it is crucial that they are utilized in ways that
minimize the negative impacts on the system. After all, in spite of strong conservation efforts in
the aftermath of barrier removal, some factors of the system may not recover for a long time, and
some may never return to the way they were at all.
References
Stanley, E., M. A. Luebke, M. W. Doyle, D. W. Marshall. 2002. Short-term changes in channel form and
macroinvertebrate communities following low-head dam removal. Journal of the North American
Benthological Society 21: 172-184
Doddl, H.R., J. R. Baylis, L.M. Carl, J. D. Goldstein, R. L. McLaughlin, D. L. G. Noakes, L. M. Porto,
M. L. Jones. 2003. Low-head Sea Lamprey Barrier Effects on Stream Habitat and Fish Communities in
the Great Lakes Basin. J. Great Lakes Res 29: 386-402
Lucas, M. C., D. H. Bubb, M. H. Jang, K. Ha, J. G.Masters. 2009. Availability of and access to
critical habitats in regulated rivers: effects of low-head barriers on threatened lampreys.
Freshwater Biology 54: 621-634.
Dean, J., D. Edds, D. Gillette, J. Howard, S. Sherraden, J. Tiemann. 2002. Effects of Lowhead
Dams on Freshwater Mussels in the Neosho River, Kansas. Transactions of the Kansas Academy
of Science 105: 232-240
Beasely C. A., J. E. Hightower. 2000. Effects of a Low-Head Dam on the Distribution and
Characteristics of Spawning Habitat Used by Striped Bass and American Shad. Transactions of
the American Fisheries Society 129: 1316-1330
Evans, J. E., J. M. Huxley, R. K. Vincent. 2007. Upstream Channel Changes Following Dam
Construction and Removal Using a GIS/Remote Sensing Approach. JAWRA Journal of the
American Water Resources Association 43: 683-697