You have the following options:

1. In your custom plugin, register an event handler as on example below

elgg_register_event_handler('update', '<entity type>', '<my_event_listener>');

function my_event_listener($event, $object_type, $object) {
    // add code for notification on river
}

2. Unregister the "pages/edit" action and register your own, so you can include any changes you wish.

But I should suggest the 1st solution