An emphasis is placed on the importance of understanding the processes responsible for resistance to flow and on the necessity of critically reviewing the assumptions and limitations associated with the methods that are commonly adopted to evaluate resistance to flow in gravel-bed rivers.A commonly applied technique is first presented for estimating the local effective boundary roughness,ks, by the use of data from a vertical velocity profile. Then methods of estimating bulk measures of resistance to flow in a river reach are outlined. Resistance equations in terms of the friction-factor parameter,,are classified into three main types: dimensional power equations, nondimensional power equations, and nondimensional semilogarithmic equations. The results of seven independent studies are considered when recommending tentative equations for each of the three defined classes of resistance equations. The bulk flow equations are considered to be applicable for the case whered/D50is greater than 10, there is no bed-load transport, and the flows are in-bank flows. Finally, a brief summary of the problems of estimating resistance to flow for an ice-covered channel is presented.Key words: resistance, flow, hydraulics, rivers, gravel-bed, velocity, friction factor, equations, Manning, Keulegan, classification.