CSS Middle Div and Footer Div Problem

[dwpegues]

Hello. I have no idea how to fix my problem, and I am hoping someone would be kind enough to help me.

I have set up my template to have a wrapper div with a top, middle, and bottom div inside.

What I need to do is have the footer (bottom div called (wrapper_bottom) move down when the content in the middle div (wrapper_middle) has more content added. Currently, when content is added to the middle div (wrapper_middle), it pushes the bottom div content out of the way. Maybe I have the divs set up wrong or maybe have them listed in the html file incorrectly?

I have uploaded a zip file of my files here:
www.dpegues.com/site_help.zip

It is also attached to this thread posting.

It contains my site images, the template in the folder called Templates, and the css file.

Thank you everyone.