The headword (evidently quoted from somewhere) and the gloss are in the genitive singular. Herodian and other grammarians give nominative and genitive together, and also confirm that Hypsizon is a river name.
Its location is unknown. However [NF], this may be a reference to Forum Traiani in Sardinia, of which the earlier name was Hydata Hypsitana; see Barrington Atlas map 48 grid A3. The city is set up in the hills (hypsi = high) and although the nearby river is called Thyrsos, the name 'Hydata' (waters) may have caused the river reference. Alternatively (and much more tenuously) there are 2 River Hypsas in Sicily, and Strabo 8.3.25 mentions a River Hypsois in Elis-Triphylia.