When a river flows over a swamp why does the discharge decrease?

Question

eg;the river Nile flows over a the sudd swamp in sudan and the velocity decresesbut why?? AND when two tributaries meet (the blue nile and the white nile) discharge increases again but why?? AND when it flows over lake nasser and through the aswan dam discharge decreases again but why??

Answer 1

because wetlands (swamps) act like big sponges and they soak up all the water. that's why we have worse floods in areas where the natural wetlands have been paved over because there aren't wetlands to soak up the water anymore.