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The vast majority of hydrological stations are set up such that discharge can be estimated from the measurement of the sole water height or stage above a local datum. Hydraulics laws show that in the right conditions there may be a unique and stable relationship between stage and discharge, which can be described by a rating curve. Because water stage can be quite easily measured on a continuous basis, the unique relationship between stage and discharge is at the basis of what is known in hydrology since the late 1800s. This article evaluates the uncertainties induced on instantaneous flow and cumulative annual flow volumes by the use of rating curves in small streams. For that we used reference data in upland and lowland watersheds where flow was calculated from stage and velocity data obtained by Doppler flow meters installed into trapezoidal wooden flumes. Rating curves were obtained by random sampling of the reference flow and stage data. Flow computed using the simulated rating curves were compared to the reference ones. For the four watersheds tested, errors did not decrease significantly when more than 20 manual gauging were performed. Significant under- and overestimations were found reaching -10 to +20% compared to the reference cumulative flow in a lowland station. The choice of the type of rating curve chosen had very significant effect, which is seldom reported.