The route that the great sarsens were brought from the Marlborough Downs to Stonehenge is still debated. This blog was started to investigate it and it is still of interest.
I am using 40 tonnes as the maximum weight of an untrimmed sarsen that was moved. The raft or sledge it was moved on then adds to this weight.
The idea that they were brought down the River Avon from Upavon to Amesbury is still popular despite the impossibility of it.
The River Avon we see now isn't the same as it was in Neolithic times, the main difference is that it has historically been dredged, over-widened and impounded in many places due to past river management.
It would have been a multichannel meandering stream in a marshy valley bottom with trees and bushes growing over and in it.
1) They weren't slid on the frozen river.
The River Avon is largely fed from ground water along its route, water that is referred to as warm during the winter as it stays at near constant temperature all year round. The river doesn't freeze solid and according to Gold's Formula the ice would need to be over 500mm thick
The River at Amesbury has a normal maximum depth of 700mm and is usually half of that, and at Upavon it is a lot less.
2) They weren't loaded onto rafts and floated down.
To float 40 tonnes of stone 40 cubic metres of water must be displaced. If we had a weightless box to load the stone on it would need to be 4 metres wide, 20 metres long and 500mm deep, the depth of the river being a limiting factor here.
A wooden raft made of logs would need to be much larger. Wood has a specific gravity of 500 - 800 kg per m3 . The photo shows how only half of a log raft is out of the water. At the lightest end the raft would need to weigh another forty tonnes to displace enough water, at a more realistic weight for hardwoods of 750kg/m3 it nearly 100 tonnes. A forty five meter by five meter wide raft at 500 mm deep. The river is not big enough.
Only by using hollowed logs or canoes could the weight and size of the raft be reduced but would they be strong enough for the massive stones?
Dragging heavy stones through an overgrown bog didn't happen, they went overland away from the river.