How about "Broken Lance" with Spencer Tracy and Richard Widmark. If I remember right there is a scene that has them going up to a mine that is polluting the water and not allowing it go down stream and the cattle are dying. Plus you could have mixed up Spencer Tracy with a older Duke.
On a side note, this is a very good movie. It also has Robert Wagner, Katy Jurado, and Earl Holliman starring it the film as well.