The stretch of the Thames that flows through Oxford is known as “The Thames or Isis” The Thames bit is easy, it probably means “dark” as when all said and done it does look pretty murky. Funnily enough the river these days is chemically very clean, supporting a decent variety of coarse fish; pike, roach and perch. The only slight problem is nitrates from fertiliser leaching from agricultural land to the north of Oxford. The reason that the water is murky is just that it runs over clay. It is this clay that allows the Thames to meander all over the flood plain, making small islands and creating a multitude of little channels – have a look at the map.
The Th at the beginning of the name was a poncey bit of classics added in the Renaissance to make it look Greek. Oxonian classical scholars would love the name Isis to be derived from a divine connection with Egypt. Alas no, it seems much more likely to be a corruption of the Latin “Thamesis”. The British are very adept at corrupting foreign words. Like the name of the centre cross road of our town which changed from Norman French “Carrefour” to Carfax, so “Thamesis” became Thames or Isis. Boring but likely.