If you are unable to see the shape of the bottom and banks, please estimate. You can base your estimate on the flow of water. The slower the water in the middle of the stream, the flatte the bottom. The particles are fine. The spaces between the particles hold a lot of water, making the sediments behave like ooze.
Sand is made up of tiny particles of rock.
It feels wonderful underfoot. The majority of rocks on this type of stream bottom are between 2 and 10 inches. The average size is about that of a grapefruit. Most of the rocks on the bottom will be large, greater than 10 inches. This kind of stream bottom is solid rock.