Elisabeth Cianciola and Sean Zimmer
Geology112
Professor Geiss
10/18/08
The Geological Impact of Dams on Rivers
Over the course of human history, development has originated along rivers because of the
benefits they provide in terms of transportation, irrigation, food, recreation, and so forth. More
recently, humans have developed the ability to manipulate waterways to create desirable water
resources through the construction of dams. However, in many instances there have been
unintended consequences of such actions. Natural river processes that transport sediments and
shape landscapes have been altered by man-made dams. This has had far-reaching affects in
terms of watershed management. In order to understand this problem, we must examine dam
structure, weigh benefits against costs of dam construction, and combine this with human
knowledge of typical geological and fluvial processes.
Why Build a Dam?
There are a number of reasons that have motivated humans to construct dams on rivers.
In order to establish reliable sources of clean drinking water, hydroelectric power, and flood
control, dams have been constructed on river systems around which humans have intended to
settle. Reservoirs for drinking water are built so that water can be extracted and diverted from
reservoir bodies as needed to meet human demands for clean water to consume and use in daily
activities. Containing drinking water in reservoirs also is conducive to managing the water
treatment process. Hydroelectric dams are best constructed on rivers that have a high drop in
elevation. Water is contained behind the dam wall in a reservoir. Gravity causes water to fall
through an intake into a penstock, which leads to the turbine (Hydroelectric Power: How it
Works, 2008). The water pushes the propeller of the turbine which causes a metal shaft in the
generator to turn (Hydroelectric Power: How it Works, 2008). As riverside towns have expanded
and become modern urban areas, floodplains and suburban neighborhoods have been competing
for the same land. As a result, dams have been constructed to direct water away from developed
areas and prevent extreme fluctuations in river flow.
Dam Structure
There are two distinct designs commonly used in dam construction: concrete dams and
fill dams. Concrete dams use concrete as their primary construction material to retain water.
Gravity dams, arch dams, and buttress dams are all types of concrete dams. Gravity dams are
suitable when the river bed is composed of hard rock and the crest of the dam will be relatively
high in comparison to the height of the dam. Buttress dams require the work of skilled laborers
and use less concrete, but are more susceptible to weathering. The arch dam uses even less
concrete than the buttress dam, making certainty in the ability of the rock to bear pressure an
absolute necessity if severe damage to the structure is to be avoided. (Durham University, 2008)
Fill dams are constructed as earthen embankments, rockfill dams, or composite dams.
Earthen embankments hold up well against weathering. Rockfill dams work well in situations in
which the ability of the rock bed to withstand pressure is uncertain. Composite dams utilize the
structural techniques of several different dam types. They are the perfect unison between art and
engineering. Overall, the availability of the required construction materials in the region in which
the dam will be built is another important factor considered when the type of dam to be
constructed is selected. (Durham University, 2008)
What Role Do Rivers Normally Play in Geological Processes?
When dams contain water, they also contain the sediment that is carried in the water. In
upstream regions where water velocity is high, rivers have the ability to pick up and transport
weathered sediment downstream. This is known as erosion. When water flowing into a reservoir
reaches a dam, its velocity decreases and the sediment it was carrying falls out. This creates a
depositional environment on the river bed. Normally deposition would occur on flat banks,
meander bends, and sand bars. (Pizzuto, 2002)
Rivers are also a crucial player in the process of forming sedimentary rocks. Water is an
excellent medium for sorting rock materials. Fine grains can be carried by water of varying
velocity, but coarse-grained sediment can only be carried by the most powerful surges in river
currents. As a result, coarse-grained sediment travels along the center of the river channel where
velocity is highest. Chemical weathering in river environments produces sediment composed
primarily of quartz and feldspar, which is deposited in river deltas. Sediment that is both
weathered and sorted is classified as mature sediment and is typically found in the reaches
furthest downstream in a river. As sediment accumulates, layers of sediment are buried.
Increases in temperature and pressure cause these sediments to be lithified into sedimentary
rocks. (Marshak, 2008)
How do dams change fluvial patterns?
Fluvial processes operate on a timescale of decades to centuries. In the case of rivers,
dams typically produce an effect known as flow homogenization. What this means is that river
dynamics are significantly reduced. For example, variation in average flow rates is reduced, the
extent of maximum and minimum flows is reduced, and there is less distinction between flow
patterns in different river systems in a region (Poff, 2007). However, dams constructed on
tributaries of lower orders generally impact only the tributaries and not the mainstem
(Magilligan, 2008). Thermal stratification in reservoirs upstream of dam walls can also change
the solubility of sediments in river water and sort sediments that are deposited on the river bed
(Poff, 2002).
When dams move the depositional environment upstream, they prevent the sediment
supply downstream from being replenished. This leads to the development of incision, or a drop
in elevation of the river bed. The sediment quality downstream is also more coarse than it would
have been without the dam, because the fine grains that are usually carried in the river water,
eroded from formations upstream, do not pass beyond the dam. Furthermore, the presence of a
dam increases the likelihood of bank failure, which makes it nearly impossible for equilibrium
channel development to occur. Banks that have been in dam impoundments are permanently
altered due to the pressure exerted on them and are a different type of rock as a result. The effect
of a dam on a river can be magnified when a river is channelized or straightened, which removes
these dynamic depositional features. Even after a dam is removed, it initiates a process of
sediment diffusion in which fine-grained material that was withheld in the dam is released and
introduced into formerly coarse-grained reaches. Scientists remain uncertain as to whether the
sediment then travels like a translational wave, having short-term, intense local effects or
whether the material is evenly dispersed throught the downstream reaches (Pizzuto, 2002)
How Do Dams Influence Watershed Management?
A dam in and of itself is a form of watershed manipulation to create benefits for human
society. These benefits include a reliable source of drinking water and water as a resource for
agriculture, recreation, and flood control. Dams change where drainage basins are located on a
landscape and thus affect what local water inputs are grouped into the same water resource. An
often misunderstood element of watershed management is the relationship between groundwater
and surface water. Groundwater can leach into surface water in rivers due to its accumulation
during rain events. Groundwater accumulates during rain events because of the prevalence of
impervious surfaces. Dam engineers need to consider this type of fluctuation in river height in
order to design dams that are effective for the cases they are handling.
Conclusion
Problems arise in nature and in human society because of dams due to the fact that they
1) alter natural processes and 2) are often poorly managed by humans. This has lead to
development of a new water ethic; a mentality by which the efficiency of water use is improved
outside of structural means (Poff, 2002). A precautionary approach in dam construction is also
recommended in that dams that allow for adaptive management and are reversible (i.e.
removable) are favored (Gregory, 2002). Regulating dams in public policies also poses
challenges. Vague terms such as “large” dams are often used and can mean different things to
different stakeholders. Land use of property including dams can also change the effectiveness of
a dam. For example, a landowner may wish to build a road or a bridge that may also interfere
with geological processes to the detriment of the natural landscape. In addition, dam removal is a
costly, resource-intensive, yet inevitable process that has formerly not been considered during
dam design. A lack of clear communication between design engineers and construction
management often mitigates any benefits that would have been reaped from forethought with
respect to dam removal as well. The ambient climate should also be taken into consideration
when designing a dam, because rain and wind are driving forces of erosion, and dams collect
products of erosion in reservoirs. As a result, dams in wetter climates will require more active
maintenance or structural features that will support the accumulation of eroded sediment. Frost
and thaw can also threaten the integrity of a dam’s structure. The seismic zone in which a river is
located even further restricts its potential in terms of dam construction.
We already acknowledge that water is an essential resource for human society by the very
fact that we try to change its cycling mechanism. However, the impacts of dams are felt severely
by other living species upon which humans depend. Many benthic invertebrates require habitats
that feature specific grain sizes, turbulence levels, water temperatures, etc. which no longer form
in the same locations along a river when a dam is built (Magilligan, 2008). Fish species often
migrate between their normal habitats and seasonal breeding habitats. Their mobility between
these sites can be restricted by dams, but topography may not produce comparable sites of both
types on one side of the dam. The impacts dams have on the survival rates of species at lower
trophic levels naturally impact predators at higher trophic levels and competitors at the same
trophic level over time. Although dam construction is motivated by human needs and interests, it
is important to consider the environmental impacts of dam construction if mankind is to avoid
creating new human needs as a burden to future generations in the process.