Flooding lakes in geography

Question

Came across the following problem on a question sheet. At its maximum level following deglaciation, approx 11,000 years ago, Lake Melville flooded the Sebaskachu River valley to an elevation of 135m above sea level. However the upper reaches of the Mulligan River and its tributaries were not flooded by Lake Melville. The tributaries of the Sebaskachu River above 135m elevation were not flooded also. What differences in river geomorphology exist between the higher river systems and the Sebaskachu River below 135m?

I know river morphology isused to describe the shapes of river channels and how they change in shape and direction over time. It is a function of a number of processes and environmental conditions, including the composition and erodibility of the bed and banks.Erosion comes from the power and consistency of the current, and can effect the formation of the river's path

Answer 1

I've never heard of the 'Mulligan river', and neither, apparently has Google Earth, (apart from the Australian Mulligan). However, there are some obvious features of the Sebaskatchu River on Google Earth that one might note. As background, 11,000 years ago the Earth was just beginning to emerge from the last ice age, so ice cover was very much greater, and sea level was very much lower, so I assume that the +135 metre lake level to which you refer is relative to today's sea level, consisting of a temporary glacially dammed lake level. These ancient lake shore lines are a common feature of glaciated terrain, as may be seen in Alaska, Scotland, Norway and New Zealand.

All lakes are relatively short-lived in geological terms, being effectively sediment traps. Sediment accumulation in the lower Sebaskatchu river basin has been intense, and has all but filled the entire lower basin. This is most obvious from the extreme river meandering, which occurs only wherever there is a high sediment load combined with a low hydraulic gradient. There are two phases of sediment fill - a general fill over a large area, and a specific small lake fill which you can see about 6 km (as the crow flies) from the river mouth, and for about 8 km further upstream.

You may note that none of the lower basin drainage is guided by the bedrock, whereas the upper basin, presumably above your threshold of +135m, is of a different style, This upper drainage is partly guided by the underlying rock structure, i.e. along strike, and partly by a cross-cutting component. The latter is most likely 'superimposed drainage', which has cut down through ice and glacial drift into the bedrock.