Hi Stix,
Bit difficult to say not inspecting the site in person but the attached is my best guess. The main channel would likely act as a natural sluice box during peak flow, especially if the basement material is irregular. The gully to the west may hold a few stones if the sapphire is shedding directly from the basalt above. Be sure to take a probing rod to sound out the gravel 'crunching' sound beneath the sand on your next visit.
Cheers,
GR