By Mike Nardacci
The Enterprise — Michael Nardacci
Flocks of ducks gather in a karst pool near Ravena, offering rest and food in the harsh weather conditions.The air temperature was hovering around 10 degrees and the highs had stayed in the teens for days. At night in this part of New York State temperatures were falling below zero and in higher elevations double-digit, below-zero readings were reported.
The forests and fields of southern Albany County were barren and frost-locked; in most places there were only a few inches of snow on the ground — and a lot less where the frigid winds had blown away whatever snow had fallen, leaving the remnants of dead plants encased in frozen mud. Shining wanly through icy clouds, the sun cast cold, pale light on the landscape, leaving no doubts that it was deep winter.
Every pond and pool was frozen over, and the few breaks in the ice encasing streams showed bitterly cold, black, churning water, moving with a sound like the shattering of glass.
And so the sound of a crowd quacking ducks happily swimming through open water, dipping or diving now and then to feed, or dropping out of flight and splashing in to join their companions, was a genuine surprise. They seemed to have no concern about the frozen, desolate ground around them or the numbing wind: They had found open water and it offered not only a haven from the bitter temperatures, it had food.
It was a karst pool, and, even in the deep freeze, its waters remain above freezing. Minnows and water bugs and a lonesome frog may appear to a patient observer and large areas of the surface and the shallow mud floor beneath may feature extensive mats of watercress. While an occasional patch of thin ice may drift along its surface like a floating sheet of black glass, the pool will remain mostly ice-free until the onset of spring further warms its waters and brings new growth on its shores.
The Enterprise — Michael Nardacci
Masses of watercress, a delicate, lacy green in the foreground, grow on and under the surface of this karst pool on the edge of Joralemon Park near Ravena. The bare branches of nearby trees are reflected on the rippled surface of the water and the icy edge at top.And yet, a few hundred feet away even smaller ponds may be frozen over to a depth of several inches. If there is enough snow, these frozen ponds may be almost indistinguishable from the rest of the wintry landscape, betraying their presence only with the absence of the remnants of the previous year’s growth — weeds or cultivated crops — sticking up through the snow pack.
So what allows one pool to remain an open haven for plant and animal life both on and beneath its surface while another is as frigid and seemingly lifeless as a feature on one of the icy moons of the giant gas planets far from the sun?
Karst.
Karst seems to have derived its name in the 19th Century from the Karst Plateau, a region of what used to be called Yugoslavia and is today a part of Slovenia. Geologists noted that the bedrock there was mostly limestone, and that acidic waters that fell from the sky (picking up carbon dioxide from the atmosphere) or formed in pools on forest floors also rich with CO2 derived from rotting vegetation, had eaten away at the bedrock.
This process results in the formation of vast caverns — subterranean stream systems — and surface features such as sinkholes, disappearing streams, and springs. And, although the type of locality is far from New York State, karst landscapes cover about 20 percent of Earth’s surface and locally make up large areas of both Albany and Schoharie counties.
Liquid water has a high specific heat, which means, essentially, that it takes a very long time to heat up when exposed to a heat source, and a very long time to cool down when that source is removed. That is why that cup of scalding hot coffee you have been served may still be too hot to drink 10 minutes later. And it is why local lakes such as Warner and Thompson may remain unfrozen even after weeks of sub-freezing temperatures and why some years a large body of water such as Lake George may not freeze over at all.
When water sinks into the ground to collect in the water table or flow through a cave, the ground above acts as a natural insulator, with the result that the water and cave will assume the average temperature of the landscape above. In the Albany-Schoharie region, if one were to average all the highs and lows of the year, the number would fall between 46 and 50 degrees Fahrenheit, depending upon the elevation at which the readings were taken.
As most of the caves in these areas are at relatively high elevations —hundreds of feet higher than sea level (at which the City of Albany is situated) — the air and water temperatures in our caves tend to fall around 48 degrees, with only minor fluctuations throughout the year. Hence, a cave that might seem a cool refuge on a sizzling hot day in the summer may seem toasty warm on a day in the winter when air temperatures flirt with 0.
But, of course, this latter impression can be highly misleading because that 48 degrees seems warm only in comparison to the frigid outside ambient temperatures. Still, the difference between cave temperatures and outside temperatures can be enormous in the winter and, in places where vertical surface cracks and fissures extend downward into a cave, the much warmer cave air may rise toward the surface.
The Enterprise — Michael Nardacci
Rime ice cakes a small opening into a cave as the water vapor in the warmer air freezes on contact with the much colder surface temperatures.Warm air is capable of holding much more moisture than cold air, and so on days when outside temperatures fall to single digits or lower, these cracks and fissures may be coated with rime ice as the warm vapor instantly solidifies on contact with the frigid outside rock and air.
The 48-degree temperature of the cave stream is 16 degrees above the temperature at which water freezes and it can take a long time to give up its heat and solidify. Therefore, when water flowing through a cave reaches its resurgence point — the place where it finally comes out of the cave passage and again flows over the surface — it may not freeze for a long time as it flows toward sea level.
And, if that water happens to collect in a pool near the resurgence, it may remain liquid throughout the winter as the water that exits the pool is constantly being replaced by additional flow from the cave.
Hence, open karst pools like the one on the edge of Joralemon Park near Ravena — easily visible from Route 102 — not only remain open throughout the winter but permit the continued growth of hardy water plants.
The karst pool near Joralemon Park is fed by water emerging from Hannacroix Maze Cave within the park, and it features masses of watercress, the dark green of its foliage defying the harsh weather conditions around it, and occasional patches of duckweed.
Karst landscapes are often invested with a certain romantic quality, given the presence of streams that flow briefly over the surface and then vanish underground, ominous-appearing cracks and pits in the bedrock that may drop dozens or hundreds of feet into blackness, and extensive cave systems — universally evoking mystery and awe.
But karst pools with their relatively warm waters sustaining many types of plant and animal life do not promote somber thoughts or fear: They show the tenacity of living things and offer promise of the eventual passing of winter and the return of new, invigorated life with the coming of spring.