Hi, 

Are you referring to the river rock being used as a dry stream, in a drainage area that is getting washed out with heavy rain, or from a downspout on your house? 

Typically, the landscape or weed barrier fabric is used under rock to keep the rock from eventually sinking into the soil. The issue with the fabric is that, eventually and inevitably, organic material will build up under the rocks and weed seeds will get in between them, germinate, and then possibly break through the barrier, making it difficult to pull them. If it is a small area, you could try hardware mesh or heavy-duty window screen, which would allow more water to seep into the soil; however,  it may not prevent the weed seed issue. 

As long as the river rock isn't being used as "mulch" around the plants, the use of landscape fabric is what is needed for the application. 

We typically don't recommend using it around trees, shrubs, and throughout garden beds because of the impermeability of water to reach the roots of the plants. As plants grow, the fabric can impede their growth by girdling around the base or trunk, and also, the same issue occurs with weed seeds settling and establishing through the fabric eventually happens.

Let us know if you have further questions.

Emily