I believe JoyceM has the main part of it. As reservoirs silt up vertically, there is less vertical in a shallow tropical lake. Maybe more sediment load too. I understand most of Bangladesh in delta sediment.

I was at a talk in school discussing the Aswan Dam in Egypt, which has cut off the nutrient-filled sediment from central Africa, along with several other issues. They said it too was rapidly filling with sediment, although a deep reservoir.

The speaker discussed a deep-location sediment outlet tho I have never heard of anyone building one- under lots more pressure at the bottom of the res, much more difficult to build...