Complex sentence: It is unusual in that it's northerly outflow is into the Glaisnock Water, flowing into the Lugar Water, the River Ayr, finally emptying into the Firth of Clyde, whilst the southerly outlow runs into the River Nith and thence into the Solway Firth.
Split into simpler sentences: It is unusual in that it's northerly outflow is into the Glaisnock Water, flowing into the Lugar Water, the River Ayr, and finally into the Firth of Clyde. The southerly outlow runs into the River Nith and thence into the Solway Firth.