A
dam built on a stream obstructs the migration of
fish and other aquatic creatures. It also slows down the
water, causing sedimentation behind the dam as the water
drops its load of sand, dirt, pebbles, nutrients and potentially
pollutants. This can cause an imbalance in nutrient load -
too many above the dam and too few below. The slow-moving water also has more time to
sit beneath the sun and become warmed, which decreases its
ability to store oxygen. Less oxygen in the water can
stress and kill aquatic organisms in the stream.
Sedimentation
in a stream can cause the water to become turbid (not
clear), which makes it more difficult for aquatic
organisms to catch prey. Siltation also can cover and kill
the eggs of aquatic organisms. How does sedimentation
happen? Sand from nearby roads can travel down into the
stream, causing sedimentation, or, fast-moving stormwater,
flowing from the street or perhaps from a nearby parking
lot, can erode the streambank as it flows into the stream,
causing erosion and siltation. Likewise, a dam can cause
water to slow down and drop its load of silt, sand, etc.
Application
of road salts during the winter affects
streams and their aquatic organisms, too.
Removal
of native vegetation (including mowing) from the edges
of a stream reduces shading of the water, thereby causing
the water to warm up and not be able to hold as much
dissolved oxygen, which can stress or kill the organisms
in the stream. Also, when there is less vegetation
bordering a stream, water runoff from the surrounding
landscape is not as well filtered when it finally flows
into the stream. Therefore, the stream receives more
pollutants. Less vegetation along the stream also means
that less organic debris falls into the water, decreasing
the food supply for small organisms living along the
streambed.
Adjacent
malfunctioning septic systems, cracked sewer pipes, or
incorrectly piped homes and businesses can leak
untreated wastewater and sewage into streams, either
through over-the-ground flow or through the groundwater.
This wastewater and sewage acts like fertilizer,
potentially leading to excessive growth of algae and
plants and then to the corresponding dearth of dissolved
oxygen as the vegetation dies and is broken down by
bacteria. Raw sewage also adds viruses and bacteria to the
water. Toxins in the wastewater also can affect the
inhabitants of the stream.
Excessive
water use and groundwater withdrawal in the
surrounding community can lower a stream's water level
significantly - even dry the stream bed, leading to poor
water quality and wildlife habitat and eventually to the
death of aquatic species.
