How does the substance in a mudflow form?

Hello the answer to the question you asked may not be what you are looking for But I'll do the best I can to answer your question.

If you are wanting to know "how does debris form in a mudslide"
then, it forms by debris getting picked up by the mudflow and being carried away by the mudflow itself.

Now if you mean "how does a mudflow form?"

I forms by a excessive amount of rain in that area/place that doesn't happen often or at all. Which causes the ground to not be used to that excessive amount of water so it causes a flooding of water. Now the mudflow happens when that area's soil is very lose so it gets picked up way to easily causing soil (mud) to color the water that dirt's color and be more thick because of the high consistency of mud in it.

                 
              Signed, Frequent Answerer Sargedog