The speed of a swimmer is $4 \mathrm{~km} \mathrm{~h}^{-1}$ in still water. If the swimmer makes his strokes normal to the flow of river of width $1 \mathrm{~km}$, he reaches a point $750 \mathrm{~m}$ down the stream on the opposite bank.

The speed of the river water is ___________ $\mathrm{km} ~\mathrm{h}^{-1}$