A significant number of estuaries in the UK not only have a high tidal range but also exhibit strong tidal currents, where tidal farms could be used for energy extraction. The purpose of this study is to examine the influence that the geometry of the channel has on the impact of a tidal farm deployed in an estuary. A hydrodynamic model (Mike21) has been used to model several idealised estuaries with dimensions based on a group of locations suitable for tidal energy extraction in the UK. The maximum changes in low and high water levels with the tidal farm have been identified for each case and located within the estuary. The changes in the time for the low and high tides in a point inside the estuary have also been analysed. It is noted that the maximum changes in water levels due to the farm range from the order of mm to a few dm and the locations of these changes are strongly dependent on the geometry of each case. In addition, the effect is generally more noticeable in the increase of low water levels and the decrease of high water levels than vice-versa. This would be associated with the loss of intertidal areas and the reduction of flood risk levels. In terms of the changes in the time of low and high water levels, the effects of the farm are negligible in all cases.