Figure 6.91 These nested basin levels are mapped with varying shades of red and vary‑ ing line thicknesses. The smaller sized basins (sub‑tributaries) are the lightest shade of red and the thinnest line. The medium‑sized basins (tributaries) are a medium shade of red and a medium thickness line. The large‑sized basin (mainstem) is the darkest shade of red and the thickest line. (This could be referred to as the porridge principle, a la Goldilocks.)