Effects of large organic material on channel form and fluvial processes

Stream channel development in forested areas is profoundly influenced by large organic debris (logs, limbs androotwads greater than 10 cm in diameter) in the channels.
In low gradient meandering streams large organic debris enters the channel through bank erosion, mass wasting,blowdown, and collapse of trees due to ice loading. In small streams large organic debris may locally influence channelmorphology and sediment transport processes because the stream may not have the competency to redistribute thedebris. In larger streams flowing water may move large organic debris, concentrating it into distinct accumulations(debris jams). Organic debris may greatly affect channel form and process by: increasing or decreasing stability ofstream banks; influencing development of midchannel bars and short braided reaches; and facilitating, with otherfavourable circumstances, development of meander cutoffs.
In steep gradient mountain streams organic debris may enter the channel by all the processes mentioned for lowgradient streams. In addition, considerable debris may also enter the channel by way of debris avalanches or debristorrents. In small to intermediate size mountain streams with steep valley walls and little or no floodplain or flat valleyfloor, the effects of large organic debris on the fluvial processes and channel form may be very significant. Debris jamsmay locally accelerate or retard channel bed and bank erosion and/or deposition; create sites for significant sedimentstorage; and produce a stepped channel profile, herein referred to as 'organic stepping', which provides for variablechannel morphology and flow conditions.
The effect of live or dead trees anchored by rootwads into the stream bank may not only greatly retard bank erosionbut also influence channel width and the development of small scour holes along the channel beneath tree roots. Oncetrees fall into the stream, their influence on the channel form and process may be quite different than when they weredefending the banks, and, depending on the size of the debris, size of the stream, and many other factors, their effectsrange from insignificant to very important.
KEY WORDS Channel morphology Fluvial processes Large organic debris