Flooding lakes in geography
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.