The two triangles are similar, so the ratio of their corresponding sides is equal. Let $x$ be the width of the river. Then, the ratio of the corresponding sides of the triangles is $\frac{x}{61} = \frac{23.4}{31}$. Solving for $x$, we get $x = \frac{23.4}{31} \cdot 61 = \boxed{46}$ meters.