On my treks, I often need to ford rivers — on my last trek (3½ day) I forded five, including four on a single day (not counting >100 smaller streams). My current practice is to take off my boots and put sandals instead, but this is a dangerous practice that several books and agencies strongly recommend against. Many rivers are glacier-fed and some are quite wide, so the risk for hypothermia is real.
Njoammeljåhkå, Padjelanta National Park, Laponia, Sweden, August 2011. To ford two dangerous steps (flow and depth were worse than they might look), or to make a 40 km detour — that is the question!
The Swedish mountain associations recommend to tie raintrousers tightly at the boots, warning explicitly that crossing on sandals carries a real risk for hypothermia. Although my boots and raintrousers are reasonably watertight and good enough to keep me dry when it rains (at least when it rains with a Swedish intensity), they won't hold standing in a (quickly) flowing river for a long time. As an alternative, the Swedish language book På fjälltur: Sarek by Claes Grundsten recommends, and I quote (translation follows):
Alternativt kan du använda vadarpåsar som träs over skodonen.
Alternatively, you can use fording bags that are drawn over the shoes.
The book goes on to state that they are for sale on the open market. I can't find information about those vadarpåsar, though. What is a vadarpåse / fording bag and where can I buy one? This forum post recommends to leave yer boots on, bring two heavy duty contractor trash bags with you, one for each leg, procede with caution, but that sounds a bit scary as well (bad maneauvability). Are there any more suitable garments, preferably at least up to the waist¹, for fording rivers? Preferably lightweight that can be packed in a small volume.
Some issues are addressed at "If I have to cross an icy, flowing river, what are some ways I can cross safely?", but the answers there do not really address the issue at hand.
¹I'm aware that one normally should not ford rivers that are deeper than knee-high, but I think one could make an exception if there is almost no flow at all.