We don't have enough information.

Dozing brush into a watercourse is not likely for a simple clearing job, nor is it consistent with pond building.
Pond building is a action you have no say over. When it fills, the amount of water will be no different on your property.

Very likely he is doing nothing wrong and you can do nothing about it.

A friend's grandfather came home from the war, bought a bulldozer and found himself in the land clearing business. In the late 1940s, he decided to create a "pond" with a creek on his property. As the story goes, a few weeks later, the Corps of Engineers came knocking and made him tear it down as they were planning a reservoir.

The land is still in their family, and while it would be fun to have a bigger private pond, using the lake a few miles down the road sure is fun.

Whether or not impounding the water is legal likely will depend on whether the water is considered to be part of the existing flow of a natural stream or river, or if it is considered to be diffuse surface water that has not entered the water body. If the watershed in question is in one of the basins governed by a watermaster program for water rights, the TCEQ's water master office should be able to give you guidance on how to determine if the water in question is subject to water rights claims or not.

https://www.tceq.texas.gov/permitting/water_rights/wr-permitting/wr_amiregulated.html