How can I fill everything below the white thing line in this image?

In the image I have attached, how can I fill the entire area below the thin white line plotted? Thank you!

Answer 1

[Matt J]

By adapting the following smaller example,

A =

     1     0     0     0     0     0     0     0
     0     1     0     0     0     0     0     0
     0     0     1     0     0     0     0     0
     0     0     0     1     0     0     0     0
     0     0     0     0     1     0     0     0
     0     0     0     0     0     1     1     1
     0     0     0     0     0     0     0     0
     0     0     0     0     0     0     0     0


>> B=cummax(A,1)

B =

     1     0     0     0     0     0     0     0
     1     1     0     0     0     0     0     0
     1     1     1     0     0     0     0     0
     1     1     1     1     0     0     0     0
     1     1     1     1     1     0     0     0
     1     1     1     1     1     1     1     1
     1     1     1     1     1     1     1     1
     1     1     1     1     1     1     1     1

[Image Analyst]

I've never heard of that one. Now I know. Thanks.