This thesis is focused on river and tidal stream power assessment, focussing particularly on uncertainties in power estimates resulting from inaccurate characterisation of bed roughness and turbine drag. It presents analytical and numerical models for the transfer of these uncertainties through to turbine power uncertainty in a strait, exploring how this affects metrics such as mean power and its standard deviation. Perturbation methods are used to determine the leading-order effects of friction and drag uncertainty in tidal stream power assessment.