Archimedes screws are an ancient pumping technology used for a variety of purposes including irrigation, land dewatering, water treatment. Archimedes screw pumps design guidelines are not well documented in the literature. Most design guidance for Archimedes screw pumps is based on empirical models and experimentation published before 1932. Current screw pump performance models do not accurately predict delivered flow rate for screws. Further, the available literature does not appear to document a phenomenon that impacts flow rate: the fill height gradient. Archimedes screw pumps are designed to rotate within a fixed trough; there is a small, intentional gap between the screw blades and the trough to mitigate wearing and friction. However, the gap introduces a leakage flow (termed “gap leakage”). As water is drawn up by the screw, it forms volumes of water between its blades (termed “buckets”). As the buckets translate up the screw, the gap leakage drains the buckets causing the volume of water, and therefore the water level, in the buckets to decrease as the buckets translate upwards towards the outlet. The fill height gradient is evident when viewing the screw since the first and last buckets have different water levels. It has been historically very difficult to measure the height and volume of water in each bucket due to the complex geometry of Archimedes screws. To address this, experiments were conducted on a small-scale Archimedes screw rotating within a clear, plastic trough which allowed the screw water levels to be viewed while pumping. The pumped water was dyed red to sharpen the contrast at the air–water interface. A camera recorded the screw during experiments. Recordings were processed to extract the fill height of the buckets during operation at various rotation speeds to quantify the fill height gradient and determine the gap leakage. The fill height gradient was greater when rotation speed was lower. The tested screw only delivered a positive flow rate when the rotation speed was greater than 8.46 rev/min. The data gathered in this novel experiment will be used to inform improvements to gap leakage modelling and screw pump performance modelling.