Deep-Spot Quest…..continued!

We left off the saga of locating the deep area of the lake with an analysis of depths taken through holes left by the ice fishing folks. That map (viewed from the north - see earlier blog item) showed 2 holes at 42' depth, surrounded toward the top (south), left (east) and bottom (north) by shallower depths, leaving the right (west) side still to be explored.
With many thanks to Bob Denoncourt for the loan of his gasoline-powered ice auger, on Thursday, March 6, we added a series of 7 additional holes through the ice that have pretty well clarified where we need to core. The augering included an initial penetration of several feet of consolidated snow and frozen slush to a 6-10" layer offering virtually no resistance just above the surface of the ice itself (a phenomenon pointed out to me by the ice fishermen earlier). The thickness of the final actual ice layer is hard to measure accurately but it appears to be at least 10" thick. It's hard to tell directly since when you finish penetrating the ice, Big Pea Porridge Pond surges up and out of the hole at you – assisted by the auger's action. When the water level settles in the hole however, if I catch the bottom of a tape measure on the underside of the ice, the water surface in the hole is at 27". Apparently the ice with its snow-slush burden presses downward on the lake's surface, forcing the water to rise up well above the level of the actual ice cover when a hole is poked through it.* It makes it hard to estimate total water depth with precision – the top of the water level in each of these holes would seem to be artificially high with water released through the opening, but the bottom of the ice is probably an artificially low estimate of depth. To make results at least consistent with one another, depths on the attached map are based on 1 ft below the measured water depth in the holes.
Anyway, five holes were bored at intervals along an imagined transect line that runs from about the Gagnon's dock to the west to the south end of the Morton's house to the east – with the westernmost hole located on a line that runs about from the cleared vertical scar on Emerson's ledge to the south to the west end of the Stettner's house on the north. A second transect line with 2 additional holes runs from the outlet to the south to the Davis house on the north. The attached map shows the adjusted depths recorded within this area. NB: I've adjusted the estimated depth at the "original holes" from the previous survey upward by a foot – using the "1 ft below the water depth in the hole" strategy described above (as opposed to the 2 ft below standard I used then).
As you will see on the map, there is an area of ca. 50 ft x 50 ft measuring a consistent 43.2 ft depth. Given the phenomenon of annual deposits of fine sediments raining down on the lake bottom filling in deepest part of the underlying lake basin over time, finding a more or less flat floor of sediment makes sense (imagine sediments settling to fill in the bottom of a tapered beer glass). Over time, with more and more sediments, one would expect the area of this flat spot to expand.
Above, I've offered visual reference lines to help you locate this area on a map or when you are paddling on the pond. Actually, you might also find the contour map available on the NH Fish & Game website (and also posted on the blog) or at the VLAP website to be useful. The problem there is that while their pond outline is accurate, their location of the deep spot and of the grassy island is badly inaccurate (for example, note the location of the grassy island in the pond visible on the Google Earth map relative to its location on the VLAP map). I've found that to more correctly locate both the island and the deep spot on their map: on a second sheet, make a tracing of the lakeshore outline from the VLAP map and line it up on top of the original VLAP map with depth contours. Place a pin through both maps at the top (north) tip of the island (= "peninsula"), to act as a pivot point, and rotate the left side of the bottom map upward by 28o. That will place the location of the island and deep spot properly to be traced onto the top lake outline map! Wonder who screwed up in making the original?



* This adds evidence to the earlier hypothesis that the standing water layer on the ice surface came from pond water (not so much from rain water) forced up through ice fishing holes. Anyone else noticed this standing water layer on top of the ice this year or in the past? Does water routinely surge up and out of holes poked through the ice (especially when NOT using a mechanized auger which exaggerates the initial surge), suggesting a release of pressure on the underlying water? The question is, does this "routinely" occur in winter, or is it the result of the record-making snowfall on the lake's surface this particular winter? Also, if this is the case, is the actual surface of the ice concaved slightly out toward the center (i.e., is the standing water layer under such circumstances deeper toward the center of the lake)? Finally, what effect might added water pressure generated in this way have on internal water movements, e.g., sub-ice currents flowing, for example, toward the outlet (or any other spot of lowered resistance, such as an ice fishing hole)? Normally, lakes under ice cover are rather stagnant since the wind contact with the surface – the usually dominant force in generating lake water movements – is sealed off by ice. This lack of circulation in winter is what often leads to low or no dissolved oxygen in deep spots because once oxygen there is consumed by decomposition of organic matter or in various other chemical reactions, there is limited water movement to replenish it and diffusion from oxygenated surface waters is too slow. ..... Lee