The frequency at which flow is measured depends on the nature of the site, whether a control structure is present, the equipment available to measure and record flow measurements, and the budget allocated for flow measurement at each monitoring location. Where measurements are taken manually, daily or weekly measurements of flow using a staff gauge are most common. If a Stevens stage recorder is available at the site, it is possible to take hourly measurements. However, the chart from the Stevens recorder must be removed and read to determine stage. Most Stevens recorders are checked weekly or monthly, hence the data from these monitoring stations is unlikely to be useful for operations. Electronic sensors, such as pressure transducers which measure stage directly or indirectly, and shaft encoders, which convert analog Stevens stage recorder output to a digital signal, lend themselves to telemetry. With electronic sensors, readings are typically taken every 15 minutes or hourly. Frequent sampling is of little consequence if the flow monitoring site is not rated regularly and the relationship between stage and discharge is not checked at regular intervals. The frequency at which this re-calibration and site maintenance is required depends on the characteristics of the site and the flow conditions. Frequent calibration is performed for stream sites in unlined earthen channels susceptible to backwater effects and where sedimentation or stream bed erosion is likely.