An uncontrolled spillway, in contrast, does not have gates; when the water rises above the lip or crest of the spillway, it begins to be released from the reservoir. The rate of discharge is controlled only by the depth of water above the reservoir's spillway. The fraction of storage volume in the reservoir above the spillway crest can only be used for the temporary storage of floodwater; it cannot be used as water supply storage because it sits higher than the dam can retain it.