The Role of Spring Thaw in String Bog Genesis

Résumé

... it is necessary to establish a more thoroughly documented framework of current physical processes operating in string bogs throughout the year. It is to this end that the present note is directed. Two problems immediately present themselves in any discussion of string bog genesis. First, there is the need to explain the initial establishment and maintenance of the ridge-hollow pattern; and second, the related problem of the concentration of string bogs in the boreal forest. ... observations near Schefferville, Quebec (54°50'N, 67°W) in the spring of 1970 are specially pertinent to explaining patterns within string bogs. Permafrost is absent from beneath these string bogs. ... There is no doubt that in the subarctic, spring thaw is rapid and debris of various kinds is transported by the meltwater over the frozen surface of the bog. Several stages in the melt-transportation process can be recognized. 1) The thaw is first evident on the thin snow cover of the bog with the development of a mixture of slush, ice and open pools. 2) Additional melt involves the removal of snow from the surface of the bog. ... 3) Pools become linked as the volume of meltwater increases with advance of the thaw. ... 4) Continued and perhaps more rapid melt of snow lying adjacent to the bog from within the spruce forests, augments the sheet flow. ... 5) With the removal of the bog ice cover, sheet flow over the bog ceases, and the meltwater becomes more channelized. ... In the bog near Schefferville where these processes were observed, incipient accumulations of the type described were only noted at the upstream end of a bog in which strings had already been developed down-bog. Where strings were already in existence, the damming effect was most apparent, although its importance declined down-bog as the floating organic debris was trapped or filtered by plants up-bog. Lines of debris accumulation are therefore considered initially to develop into permanent sites for plant growth at the downstream end of the bog, and then progressively build in an up-bog direction always at right angles to flow patterns of spring meltwater. "Younger" strings, according to this view, will occur at progressively higher elevations within any given bog. ... it is assumed that the sites of detrital deposition become the preferred sites for plant growth because of their relatively better-drained condition. ... Once started, such a process will accelerate by its own effects. ... Although excavations in the Schefferville area were not conclusive, ... the hypothesis of in situ growth of strings was supported by bog stratigraphy. ... Emphasis is placed on the primary role of organic accumulations during early phases of the spring thaw, a period of the year when few observers have had the opportunity to examine such bogs. A vital role is attributed to the more intense plant growth on initial shallow deposits which leads to the formation of the ridge. Ice-push and frost heave appear to play secondary and perhaps localized functions. Solifluction, differential settling and tilting due to permafrost melt, and compaction of peats under varying loads, are not factors which appear to be of great importance in string bog development. There remain several critical problems which require further study ....