Hi Peter, 

Human vectors, such as boats, fishermen, etc., are the main causes of biological invasions in rivers and lakes. I believe that the transposition of rivers and changes in water flow are not so frequent to be considered the major causes of introduction of invasive species in these places...

Cheers!

N

Pollution and euthrophizacion of waters (with more acuatic weeds)  could help to some to slow water flow but mainly to reduce oxigen. Thus favour to some  allien species as Cyprinidae and  Crustacea

The human vectors are the most important causes of biological invasions, and sometimes the intervention river flow by anthropogenic factors (roads, dams) can be an aid transport to the dispersion of the invasive species which had not previously report their presence.

Hi Peter. Because microorganisms as microalgae are easy dispersal, invasive species can reach even pristine places as I have already found Cylindrospermopsis raciborskii, an invasive cyanobacteria in remote areas in Amazonia.

Attributing the replacement of natives with exotics to modified flow regimes OR the loss of native species will be tough without directed experiments.  You might consider classical community ecology concepts like the diversity-stability hypothesis and ideas that disturbed communities often more easily invaded than intact communities.  In warmwater streams of the U.S. we typically see communities dominated by generalist species and non-native/invasive species in streams suffering from urban and agricultural effects, as well as in streams with modified flow regimes.  

Thank you colleagues...highly appreciate your input. @ Andrew Taylor i indeed agree that experiments will be the better way to establish the effects of changing river flow on biodiversity.however, it will be difficult to find 'pristine' ecosystems where just the change of flow and physical manipulation can be investigated.