<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def delete_river(self, river, river_name=None):
 """ Delete a river """

        if isinstance(river, River):
            river_name = river.name
        return self._send_request('DELETE', '/_river/%s/' % river_name)