I have just added this to the map of places to fly your drone at Drone Scene:
Land owner permission requirements unknown.
The River Nith is in the South West of Scotland and flows from the hills of East Ayrshire to the Solway Firth. It is listed as one of Scotland's National Scenic Areas.
There is parking by the side of the B725 with paths to the beach. Be aware of the signs about the dangers of the mudflats! I flew from the roadside on a late summer evening, bit I'd avoid this area in winter as it's an internationally important winter feeding site for waders, geese and other wildfowl.
The originator declared that this location was not inside a Flight Restriction Zone at the time of being flown on 26/08/2021. It remains the responsibility of any pilot to check for any changes before flying at the same location.