In Chapter I a derivation of the equations and an extension of the mathematical model will be carried out. The sediment-mixture is separated in a number of fractions - each with a representative grain size - and the equations describing the sediment-movement are split up for every fraction separately. In order to get some insight in the new model in Chapter 2 a restriction will take place to sediment-mixtures consisting of only two sediment-fractions. As a result only one extra dependent variable viz. the probability of one of the fractions, comes into the equations. Moreover the characteristic directions and relations belonging to the set of partial differential equations will be derived mainly in order to obtain information concerning the time-scales of changes in bedlevel and bedcomposition; also the interaction between these changes and the influence of some determining parameters will be studied. In Chapter 3 specific calculations will be carried out of the characteristic directions and relations and the influence of two possible concepts for a transportformula per fraction is studied. In Chapter 4 four simple applications of the mathematical model for two sedimentfractions are treated. The watermotion is simplified in order to be able to carry out the calculations partly by hand. In Chapter 5 a summary and conclusions are given.