Solution:

The resultant velocity of the swimmer is the vector sum of the river's velocity and the swimmer's velocity relative to the water.

Given:
- River velocity = 6 km/h
- Swimmer's velocity relative to water = 9 km/h

Using the Pythagorean theorem:

\[
v_{\text{resultant}} = \sqrt{(9^2 + 6^2)} = \sqrt{81 + 36} = \sqrt{117}
\]

Thus, the resultant velocity of the swimmer is:

\[
{\sqrt{117} \text{km/h}}
\]