On leaving Hertfordshire , the watercourse splits off into several separate branches , a few of which rejoin it .
Its main branch flows into the River Thames on the reach above Penton Hook Lock at Staines - upon - Thames .
On leaving Hertfordshire , the watercourse splits off into several separate branches , a few of which rejoin it , and its main branch flows into the River Thames on the reach above Penton Hook Lock at Staines - upon - Thames .