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August 14, 2003 3:23 PM

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River Types: Limestone Rivers

Limestone Rivers and Spring Creeks Gallery


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 severe meanders 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 food 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.

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