A scaling analysis of the depth-integrated momentum equations tailored to t
he rough bed conditions of mountain streams suggests that certain velocity
correlation terms that arise from depth integration, and which normally can
be neglected in the case of smoother alluvial channels, can be a significa
nt part of the momentum balance in these steep channels. By introducing the
kinetic energy equation of the time-averaged motion to treat these correla
tion terms, which involve products of local deviations in velocity componen
ts about depth-averaged values, a flow model that suitably characterizes st
reamwise accelerations is obtained. A linear stability analysis using a flo
w model that retains the streamwise correlation terms suggests that their e
ffect is to strengthen the initial selection of bed form wavelengths, as re
flected by sharpened peaks in curves of growth rate versus bed form wavelen
gth. Wavelengths with zero migration rate are close to wavelengths having t
he large;st growth rate; thus selection of fixed bars is strong. Critical w
idth-depth ratios necessary for bed form growth are significantly less than
the critical ratios that are predicted when correlation terms are neglecte
d. Moreover, a broader band of wavenumbers can be activated at a given widt
h-depth ratio, and bed form modes representing midchannel bars can be activ
ated in a narrower channel than would otherwise be predicted. Thus alternat
e bars can initially "compete" with midchannel bars, particularly at low se
diment transport rates. This competition probably contributes to the comple
xity of bed topography that is typical of rough, mountain channels.