Bedrock is simply the last layer of rock. When you go as far as you can go you have reached bedrock. (BED) of the river or creek etc.
In your particular case with the creek shown you have 1.Water...2.Loose rock that CAN BE MOVED by hand and 3. "BEDROCK" that CAN'T be moved by hand.This is where your gold would be settled to along with gold in the cracks of the rocks.