I suggest BD to build its own reservoir inside BD border. Think of two small height barrages across the two Meghna channels that sandwich Bhola island. 20m high barrages there and 5 or 6 metre high levies (embankments) along the two sides of this river will make a vast reservoir from Rajshahi to Bhola, and from Rangpur to Aricha.
Now, build sluice gates across all the mouths of contributories and distributories to this river. At the end of rainy season, close all these small gates and the large gates at the Bhola Barrage. It becomes a big water reservoir. Use this water to irrigate the winter crop fields.
Note that it is not possible to take this water directly through canals to all the paddy fields. However, water that is reserved in the Jamuna, Ganges/Padma and Meghna river system will also flow horizontally underground. We will not see it, but it will happen because it is a natural phenomenon.
When this happens, the underground water table throughout the country will rise. It means more water will be available at lower pump-up costs during the crop season.
Only two days ago I have read a newspaper report in Bangla that the govt is considering to build the Ganges Barrage. It will be built not in Bhola that I have suggested, but will be somewhere near Rajbari District. This is logical.
In fact, the first barrage cannot be built in Bhola because of about 35 metre difference in land elevations between Rajshahi and Bhola. It means that the riverside levies (embankments) approaching Bhola will have to be raised to almost 35 metre. This is not physically possible in a plain land.
Therefore, I retract my previous suggestion of building the first barrage at Bhola. But, I would suggest the 1st barrage at Rajbari and the 2nd barrage at Bhola.
It is also possible to build a power station at Rajbari point. This power station will produce more electricity if another barrage is NOT built in Bhola. In this case, the difference in elevations in water surface at upstream and downstream sides of Rajbari Barrage will be larger than if there is a barrage in Bhola.
If ultimately a Bhola barrage is built, it will reduce the production of electricity, but will increase the volume of water reservoir.