Reuss.

From an earlier form Rusa, first attested in 1296. Ptolemy recorded the river's Pre-Germanic name as Silana, which is reflected in the name Silenen. The Germanic name for the river is attested as Rusa or Rusia from the 9th century, and was occasionally recorded as Ursa in the Early Modern period (compare Urseren). This name likely derives from the early Germanic *Rūsi or the oblique form *Rūsjō-. Greule (1982) interprets the name as an Old European hydronym, closely related to Riß. Given Ptolemy’s record of the pre-Germanic name Silana, it is possible that only part of the river was known as Rūsi in antiquity. Alternatively, Rūsi may have been introduced as a Germanic name during the early medieval period, potentially derived from Old High German rūsa, riusa (“fish trap”) (see the modern Reuse). More at Reuss.