Debris flows and El Niño in southern California landscape

Stream order and drainage networks

Stream order ­ Geologists classify the segments ("links") of a drainage pattern using a convention in which its number ("stream order") increases as the size and number of tributaries increase. In the diagram, the steep, small segments are designated as 1 (or "first order"). Wherever two segments of the same order join downstream, the order of the downstream segment is increased by 1; two first-order drainages join to form a second-order drainage, and so on. Note that "stream order" in a drainage network is not determined by the presence or absence of flowing water, but by the shape of the land surface, which determines where flow will be concentrated when water is present.

• Debris flows tend to form on steep slopes and accelerate downslope until the flow slows and stops. The base of a steep slope is most likely to be exposed to small debris flows from small, steep drainage channels, first and second order drainages.
• Locations in and near the mouths of relatively steep, larger ravines, which are generally second and third order drainage channels, can be vulnerable to unexpectedly large flow surges if surface runoff is bulked by debris flows in the drainage basin upstream.
• Still larger drainages, such as canyons (fourth and fifth order drainages), generally have gentler gradients, and larger volumes of slurry are needed to maintain flow. During intense rainstorms these larger streams can receive both increased surface runoff and increased input of debris flows, which may cause large surges of debris-laden flood water.

Debris flow hazards and rainfall: what does heavy rainfall do?

Debris flow hazards and wildfires: effects of removing vegetation

How to prepare for debris flows

Geologic mapping in southern California

For further information, contact David Miller

http://geology.wr.usgs.gov/wgmt/elnino/scampen/streamorder.html, 29 December 1997, Contact: El Niño Web Team