Fistly, they are not 100% reed. The inside is stuffed full of empty plastic water bottles to provide buoyancy. Recycling par excellence.
Secondly, reed rots. So a boat has a life of approx 9 months before it becomes too shameful to be seen out in. At that point, they drag it onto one of the floating islands (also made from reeds), cut it open down one side to recover the placcy bottles and use next time round, fold the old boat closed and turn it over, and plant potatoes in the rotting reeds. Apprarently one old boat is good for two crops of spuds before it disintegrates.
If I could be assed, I'd find some of my pix of this and scan them for your eddification, but I can't be.
Did make me wish for a nice big powerful mobo though, cos eyeing up those floating islands gave me this incredible urge to see what would happen if given a huge dose of wash ...........joking!!!