target_guid for river items is always set to zero.. any tips on how to filter the river based on container guids?

i am looking at adding filters to the river here and would like to be able to choose to filter the river based on the container guid of the river items.
i should be able to just add the container guid to the query that retrieves the river data by way of using 'target_guids' according to the source for elgg_get_river(), but every river item i have looked at so far has a target_guid of zero... so this approach does not work.

am i missing something here? is the only way to do this to use custom joins / wheres etc. ?
thanks