do you have to make it that hard..
well, I still think they should have put "IJsselmeer" first, since it's the preferred spelling in the language the word comes from.
The (Gelderse) IJssel is river, and a 'meer' is a lake, just so you know.
There is even a province called "Overijssel" which is 'on the other side of the IJssel' - well if you're west from it and looking east..
Not so long ago the IJsselmeer was the Zuiderzee (='southern sea', it was right in the middle of the Netherlands though), but 'we' made the Afsluitdijk (closing dike?) and since the IJssel is the biggest river flowing into the lake, it was called IJsselmeer.
The name "IJssel" is very old, and
wikipeadia knows how to spell it properly. If the English spell IJssel with a capital IJ, then why not spell IJsselmeer with a capital IJ? It's the same name, but with "meer" added because it's the lake, not the river.