Ammonia Glaciers

In Three Worlds To Conquer (London, 1966), after describing oceans of ammonia on Jupiter, Poul Anderson presents some more information about this fictitious Jovian environment but I cannot fully picture what he says:

when air masses are pushed down from the poles, the surface becomes so cold that ammonia freezes;
such glaziers flow quickly in Jovian gravity;
denser than liquid, they break up and sink on reaching the sea;
in shallow ammonia, the broken glazier chunks melt quickly;
therefore, a torrent pours from the fjord where Theor, paddling on flotsam, had hoped to land so he is unable to enter;
however, by lying flat on a smaller piece of flotsam, he manages to mount a wave and surfboard to shore.

So a torrent prevents him from paddling to shore but a wave carries him there? This sounds contradictory to me but maybe some surfboarders among Anderson fans can explain how it works?