your water pressure looks to be very low if you were to place a line across the water area using sand would it stay or is the flow going to wash it away ? if it's going to wash it away. Your could shut down the pumps let the rocks dry then run a bead of clear caulking across the large flat rock and sprinkle some sand onto it and lightly push it into the calking. you can do that many ways but i would use a small 1" paint brush and tap or brush the sand into the silicone lightly repeat this a few times and you should make ripples as the water flows over your bead of silicone and sand as they are basically making damns the water flows over . if one area lets to much water out shut the pumps down let it dry and try to add more silicone to the area. experiment and enjoy the challenge for every action there's an equal and opposite reaction think of the beach the lines in the sand at the waters edge they are never straight they are always wavy but in a horizontal plane