How to override a page

Hello,

I'm trying to override pages/river.php with something like:

elgg_register_page_handler('activity', 'my_page_activity');

function my_page_activity() { ... } // In mod/my/start.php

However, this does not seem to work at all and the function is never called.

Could anyone point me in right direction for doing something like this?

Many thanks,

Chris