16. A siphon tube level control system for a reservoir having a fluctuating water supply such as an irrigation ditchcomprising a plurality of siphon tubes arranged along the bank of the reservoir, selected ones of said tubes having an inlet extending into the water in the reservoir and an outlet extending outside the reservoir and leading to a runoff area along said irrigation ditch to draw the water from the reservoir in a siphon action, float-controlled valve means associated with said ones of said siphon tubes including a valve member at the outlet of each of said ones of said tubes responsive to the water level in the reservoir to stop the flow through said ones of said siphon tubes when the water level in the reservoir drops to a preselected lower level and to automatically open the valve members and restart the siphon action in said ones of said tubes each time the water level rises to a preselected higher level to maintain the water in the reservoir between said upper and lower levels.