Since you have an okay to dig on the properties your hunting you might try this tactic...
Check the gravels and bank edges for even the smallest of flint flakes. That would be an indicator of ' man made 'activity and so it follows that better artifacts could truly be in that drainage. Use some small tools like hand rake and planting shovel to dig test holes in the drainage bed, right down to bedrock if possible.

The bad news is that if you are lucky enough to find just one worked artifact in your tests...the whole
drainage bed has to be removed and you will have to buy some new display cases.