In contrast to freestone rivers, limestone streams (see: Galleries – Limestone Streams) emanate from groundwater sources, usually as artesian aquifers (Figure 28) 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 hundreds of miles away from the river it creates. These aquatic ecosystems tend to be more stable, but less robust than freestone systems, and gradually seek lower altitudes with less severe meanders and lower volumes of water (Figure 29). Their stream banks tend to be less eroded and are usually at the level of the river, as opposed to freestone situations where the stream bank may rise abruptly from the waters edge due to the high rate of erosion and steep geological gradient. Temperatures fluctuate less during the day in these slow moving waters, due primarily to the low amount of surface area. Finally, their chemistries are quite different from freestone rivers, making them basic in pH, and thus highly productive with respect to the amount of biomass they can produce per linear mile. This is because the outer shells and exoskeletons of the macroinvertebrates living there depend upon the presence of dissolved calcium, from which they construct their exoskeletons. The more available calcium, the greater the productivity of the river. Simple food chains (Figure 30) are common in these settings, as opposed to the more complex food webs (Figures 7, 8) of freestone rivers.
Ecology of the Limestone Stream
The caller asked if I wouldn't mind speaking to their local chapter of the Federation of Fly Fishers on a subject dealing with stream ecology. One of their members met a friend of mine out West earlier that year and he told them I never refused an opportunity to talk about my favorite subject. He was right. Could I go to St. Louis to give the talk, and if I could, they would throw in a visit to a small creek to the south (just where they didn’t, or more exactly, wouldn’t say). It’s said that everyone has his price, and somehow the word got out that that was mine. The trip turned out perfectly, with great people in attendance, excellent weather, and some interesting fishing in the "little known" stream. The “limestoner” they took me to was about a three hour drive south of town (Figure 31). The water was ice cold and somewhat milky in color, due to runoff from a recent rain, and trees blown down along the riverbank attested to the fact that the storm was not typical. High winds, perhaps even a tornado, had crossed its path. Understandably, the trout were not too cooperative, readjusting to the aftermath. We managed a few 10-12 inch stream-bred rainbows that convinced us that there was nothing wrong with our presentations. I was impressed by the intimacy of the setting and the extent to which the plant life had grown within it to occupy most of the streambed. In the spring of 1993, the Great Flood down the Missouri-Mississippi drainage system changed forever this small creek and all other rivers along its path of destruction. I hope it has recovered and that the fish are once again happy to be there.
The geological term for limestone rivers refers to them as karst systems. Limestone streams are the jewels of the trout world. They are all spring-fed, usually narrow, cold, and gentle in their wanderings through gradually sloping countryside. The River Test is a karst system in the south of England. Depending upon the geography (e.g., England vrs. Midwestern USA), their stream banks may support less tree cover that freestone rivers. In general, the typical limestone river’s banks are less eroded than freestone riparian environments, since their gradients are more gradual compared to those rivers emanating in mountainous regions. Compared to freestone rivers, they have several advantages in terms of trout production. Their even-tempered, demure nature allows for in-stream macrophytes (i.e., large plants; Figure 32) to take hold and flourish. This, in turn, provides an almost unlimited food supply for the abundant invertebrate life, particularly crustaceans, such as scuds (Figure 33) and crayfish. The trout can then munch out on them at their leisure, year-round. The lack of competition for food allows for greater carrying capacity with respect to the number of fish per linear mile of stream.
Free calcium is dissolved into the water column from the erodible limestone deposits (a concentrated source of calcium carbonate), over which the stream flows. Calcium is an essential ingredient for the exoskeleton of most macroinvertebrates, but especially so for crustaceans. Calcium becomes dissolved by a leaching process. Carbonic acid forms when CO2 in the atmosphere dissolves into the water, turning it slightly acidic. Before the acid can dissipate into the water column, however, it comes in contact with the substrate, and dissolves a small amount of calcium carbonate, releasing calcium. This essential element then has the option of being taken up by the myriad crustaceans living in the abundant in-stream plant life, and becomes incorporated into their exoskeletons.
Limestone streams usually begin as underground systems, and because of that they are not as adversely affected by ambient temperature, as are freestone rivers. Their average year-round temperatures are 52 degrees 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, witness Joe Humphreys giant 26 lb. brown caught in a Pennsylvania limestoner (Figure 34).
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 wonderful places (Figure 35). 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 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.