Had not thought about albedo! Attached is an image from a quick search from Worldview, showing what seem to be sediment-rich waters issuing from the Lena delta, spreading at least 100km into the Laptev. (Algal blooms are also a possibility and perhaps these would be promoted by the nutrients in the Lena sediment). No doubt there are better images available. However, visually, these seem to increase albedo -- the seawater looks darker. This is not necessarily the case on all solar wavelengths of course.
Another possible factor is that particles in the water would absorb energy more intensely at the surface, rather than allowing the sunlight to penetrate too far. So that might result in even warmer waters right at the surface. That might offset any possible albedo increase to some extent.
slow wing raises the interesting possibility of these sediment laden waters sinking rather than floating. Aluminium, through a source from gerontocrat, provides a possible solution with the information that "Yearly average turbidity in the Lena is 43 g/m3. Maximum is 400 g/m3 in the lower reaches of the river." So, if we accept this information, and ballpark the Lena's non-turbid water as 1000kg/m3, then it is possible to make the following very crude calculation. Taking the maximum turbidity of 0.4kg/m3, that would make a cubic meter of Lena water have a density of
around 1000.4kg/m3. Sea water usually has a density of 1020 to 1029 kg/m3. So, on this limited basis, and assuming equal water temperatures, the sediment-laden Lena waters would float. This was probably Aluminium's point, I'm just parsing it out. (BTW, Turbid currents can certainly be impressively dense, but the Lena goes through a delta, and probably sheds some of its sediment there).
For me, your albedo point is a very interesting one, even if it is hard for me to tease out its effect from the current information I have.
What about Albedo?
I presume that the flood water coming down these Siberian (and North American) rivers is not a beautiful clear and bright mixture of white ice and transparent water but full up muck - sediment, organic matter (and the occasional dead animal), and being less dense than sea water a brownish layer of river water will spread over a considerable area. Albedo must surely drop, with a considerable impact on heat capture as we are in maximum insolation?.
We are talking about a considerable number of cubic kms spread out in a thin surface layer.
Any studies on this?
EDIT: I believe the fresh water from the Amazon spreads over the ocean surface out to a distance of up to 150 miles from the river mouth - peak flows from the Lena exceed Amazon flow?