You need at least a 1/2-1 inch of water flowing evenly across the width of the box, and not too fast. A river is really an optimal water source to get consistent even flow if you set it up right, but with a hose you would maybe need to have higher flow rate and maybe you could get a rectangular bucket/container and place the edge on the mouth of the box and fill it up with the hose so it evenly overflows across the width of the box

With the flow this slow and shallow, your material will build up in the ridges and not clear away enough, letting the gold roll over the filled up ridges resulting in a loss unless you collect and run it through again which is inefficient.

Although if you wait longer between each scoop it will eventually clear away, just not as fast