How to fill part of an image using imfill?

I have an image like this:

img

and I wanted to fill the bottom part using a mask like this as the location of seed points:

mask

This is what I tried but didn't work:

img = imread('img.jpg'); maskImg = imread('mask.png');
[Gmag, Gdir] = imgradient(img,'sobel');
mask = imbinarize(maskImg);
[rows,columns] = find(mask);
bw = imfill(Gmag, [rows(:) columns(:)]);
figure, imshow(bw);

How can I fill the road part of the image above?