auto height not working

[mattrd]

I have div on the left (with a green background) that I would like to extend to the bottom of the page and line up with the content div but I can't seem to get it to work. Can someone help me and tell what I am doing wrong?

this is the page:
http://crccreek.ianncomm.com/about/initiatives

this is the code i have on the div

#leftnav {
	float:left;
	width:200px;
	height:auto;
	padding-left: 10px;
	padding-top: 15px;
	font-family:Arial, Helvetica, sans-serif;
	font-size:12px;
	background-color:#E4F3D4;
	overflow:hidden;
	vertical-align:bottom;
}

if you need to see the entire stylesheet, you can view it here:

http://crccreek.ianncomm.com/css/style.css