def find_corresponding_basins(pfaf_id_level6,gdf_level7):
    """
    Using a pfaf_id from level 6, find all hydrobasins in level 7 that 
    make up the hydrobasin level 6 polygon.
    
    """
    pfaf_id_level7_min = pfaf_id_level6*10
    pfaf_id_level7_max = pfaf_id_level7_min + 9
    
    gdf_level7_selection = gdf_level7.loc[(gdf_level7["PFAF_ID"] >= pfaf_id_level7_min)&(gdf_level7["PFAF_ID"] <= pfaf_id_level7_max)]    
    return gdf_level7_selection