........................The drain is at the bottom of the waterway so that is where muck collects.
Logic, and as Symphony suggests, the bottom of the waterway is where all the "muck" would collect, but it can be surprising just how much of the stuff will be found on any horizontal surface in other areas within the cooling system. Around the tube nest within the heat exchanger is a favourite place, and this winter I have just cleaned out the cooling water expansion tank, removing a large quantity of sediment and enabling me now to see the water level within that tank once again. Oh, and by the way, this was the highest point within the waterway, where that tank was located some 30 - 40 cm above the top of the engine, and if that doesn't defy logic, then I don't know what does !!
One of my reasons, by the way, for taking that tank off was that directly below it, on my B36(2002) and in way of one of the cooling hoses there is a piece of electronic/electrical equipment located. This is something to do with the battery charging system, and has cooling fins that the hose was rubbing and chafing against. The tank when put back and after cleaning it, was moved 15cm to starboard to prevent further chafe on the hose. The edges, and particularly the corners, of those cooling fins are remarkably sharp.