Effects of experimental floods in two rivers with contrasting valley morphologies

Environmental high flows are being implemented in many flow-regulated rivers globally. The questions arise as to whether different rivers respond similarly to artificial floods, and what are the long-term ecological consequences of such flood programs? This study evaluated abiotic/biotic dynamics during experimental floods in two rivers with contrasting valley morphologies (canyon-confined vs open-valley) as well as long-term response patterns of macroinvertebrates in both rivers to 16 years of experimental floods. Both systems showed an increase in measured parameters (turbidity, total phosphorus, drift, seston) during the rising limb of the flood with subsequent decreases being observed before the falling limb of the flood (all parameters displayed typical hysteresis patterns with discharge). Contrasting differences existed in respect to immediate effects of these two floods on macroinvertebrates and benthic periphyton (decreases or no change). Over the long term, major channel changes occurred from initial floods with less change observed from later floods in both systems. Macroinvertebrate densities fluctuated seasonally, but showed no long-term response to the flood program, whereas taxa richness increased in both systems. The rivers differed in macroinvertebrate community structure and this was reflected in shifts in community assembly over the study period in both systems (being substantially greater in the system flowing through a confined valley). The results suggest that immediate flood effects are quite similar among rivers, even over multiple years, but long-term trajectories in community assembly reflect changes in local habitat conditions in respect to external drivers such as landscape geomorphology.