conditional css for footer (float problem)?

i have float left elements (a css grid image gallery) on a single page in wp. they end up making the page hang down into the footer unless i add this bit of code to the stylesheet.
#footer {clear: both;
width: 900px;
float:left;
}

this recenters the footer and fixes the problem, but then on every other page, the footer is misaligned (floating left). is there a way to apply the above css to an instance of the footer on a single page? my guess is that will require some conditional php statement, perhaps in footer.php– but this is way over my head. can anyone help me out (or give me a better solution)?

thanks!