River Types: Limestone Rivers
In contrast to freestone rivers, limestone rivers
emanate from groundwater sources, usually from unconfined aquifers
or underground rivers that course their way through calcium
carbonate deposites. Spring
creeks sometimes begin where hydrological pressure generated
from a confined
aquifer meets the earth’s surface, and forces water
up onto and over the land. In these situations, the origin of
the water itself can be miles away from the river it nurses to
life. Both of these types of rivers tend to be more stable than
freestone rivers, and gradually seek lower altitudes with less
and lower volumes of water. Their stream banks tend to be less
eroded and are usually at the level of the river, as opposed to
most freestone situations, where the stream bank may rise abruptly
from the waters edge due to the steep gradient and high rate of
erosion. Temperatures fluctuate less during the day in these slow
moving waters, due primarily to the low amount of surface area
and the available underground source of cold water. Finally, their
chemistries are quite different from freestone rivers, making
them somewhat basic in pH, and thus highly productive with respect
to the amount of biomass they produce per linear mile. Simple
chains are common in these settings, as opposed to the more
complex food webs of freestone rivers.
Limestone streams usually begin as underground
systems, and because of that they are not as adversely affected
by ambient temperature, compared to freestone rivers. Their average,
year-round temperatures are more connstant, averaging 52°F.
Bank-side vegetation is of little consequence to maintaining stream
temperatures or to the flow of energy, due to the large biomass
of in-stream macrophytes.
If you’re a trout, it’s a rather nice place to live.
Record-size fish have been captured from limestone streams.
What human activities pose the greatest threats
to these fragile aquatic habitats? In the limestone streams of
eastern Pennsylvania, for example, encroachment
from housing developments and shopping malls (more accurately
dubbed “mauls”) are the main problems. By drawing
off too much groundwater from the surrounding aquifers, they lower
the water table, and slow the flow rate of the nurturing springs
and underground rivers that supply water to the limestone stream
above. The result of slower currents is a warmer river during
summer months, and this altered environment selects for plants
of the wrong kinds; both negative conditions reduce the level
of dissolved oxygen and threaten the stream’s inhabitants.
Nutrient loading from adjacent farms, in which dairy cattle are
allowed to graze along stream banks and defecate directly into
the stream poses yet another series of threats to the well-being
of these small wonders. Fortunately, situations such as this one,
at least in Pennsylvania, have been addressed in favor of the
rivers. An important first step to their rehabilitation began
by informing farmers living next to these fragile environments
of the proper ways to create barriers, preventing cattle from
accessing the river.
Without greater appreciation for these delicately
balanced ecosystems, further
damage to them due to human activity is certain to continue. A
consensus from the public in favor of reversing the damages is
what is needed. By becoming increasingly aware of the connectedness
of our lives with the natural world around us, we cannot help
but improve the lives of the inhabitants living next to us. Renewal
of life is key. Damaged river ecosystems struggle to renew themselves
each season, often failing to do so as the result of too much
human interference with the natural processes that support them.
Our task is to help insure that the life forms in the river not
only survive there, but thrive.