Every factor you have listed plays a role in the morphological description of river floodplains (do not forget vegetation cover). To compare the relative contribution of the different factors:

first: one should know which morphological aspects should be analysed (floodplain elevation? levee formation? local bed erosion? sedimentation rates? river planform?) as well as the time scale (short-term vs. long-term);

second: each factor should be quantified (how much sediment mining and where on the river floodplain?);

third: the factors interact with each other and their effects on the river morphology are the results of their non-linear interactions.

Conclusion: it is impossible to provide an answer to you question, because it is too general.

Dear Alessandra

Thanks for your clear answer. Actually in my thesis results i have STATISTICAL relationships between  land use change and morphological parameters of river (active channel) in decade scale. Also i have some information about the gravel mining sites (especially in braided reach). I talked with some geomorphologist and they say that the most of this morphological changes are related to gravel mining sites. By the way i just wanna to know more ideas about main factor in my study site.

Info and suggestions from Dr. Crosato appropriate.  You might check out Dave Rosgen website on stream types, valley types, measurements, sediment supply, successional change due to channel adjustment.  There is a difference among stream type and how they respond to sediment and flooding. Both gravel and sand mining in the channel as well as stream straightening and ditching (hydrologic modification) can be very disruptive.  However, that being said, mining within the annual budget of sediment supply may have some positive benefit to maintain a single channel, rather than braiding from sediment overload.  There is a difference in braided channel and anastomosed, one is unstable, one stable.  The morphological changes through time is what one would expect from a few severe activities and effects initially, and then more activities with somewhat better controls for one reason or another, which suggest the braided section may be gaining to rechannelize.  

Www.wildlandhydrology.com

 What is with anthropogenic constructions (levees, dams), river regulation and channelization and natural climatic changes?  

Dear Saleh, 

you have a very interesting case with clear river narrowing with time. It looks that the observed changes can indeed be due to sediment extraction. However, to say something more about the cause, it would be important to see also the temporal changes in longitudinal bed level profile, basically to observe if the river is incising and how much and in which reach. Information on the composition of bed material would also be very important. I do not study these processes (morphological changes) statistically, but with a physics-based approach, because I think that this is the right way to detect the real causes. I attach my lecture notes (I teach river morphodynamics) where you can find some information on the effects of interventions on the morphology of rivers. The lecture notes focus on sand-bed rivers, but the basic concepts are there. I hope you find this material useful. 

Alessandra Crosato