▁can ▁also ▁act ▁er os ively ▁as ▁they ▁traverse ▁a ▁surface , ▁in ▁a ▁process ▁known ▁as ▁tra ction . ▁ ▁Bank ▁er os ion ▁is ▁the ▁we aring ▁away ▁of ▁the ▁banks ▁of ▁a ▁stream ▁or ▁river . ▁This ▁is ▁distinguished ▁from ▁changes ▁on ▁the ▁bed ▁of ▁the ▁water course , ▁which ▁is ▁referred ▁to ▁as ▁sc our . ▁E ros ion ▁and ▁changes ▁in ▁the ▁form ▁of ▁river ▁banks ▁may ▁be ▁measured ▁by ▁inserting ▁metal ▁ro ds ▁into ▁the ▁bank ▁and ▁mark ing ▁the ▁position ▁of ▁the ▁bank ▁surface ▁along ▁the ▁ro ds ▁at ▁different ▁times . ▁ ▁Th erm al ▁er os ion ▁is ▁the ▁result ▁of ▁mel ting ▁and ▁weak ening ▁per ma f rost ▁due ▁to ▁moving ▁water . ▁It ▁can ▁occur ▁both ▁along ▁rivers ▁and ▁at ▁the ▁coast . ▁Rap id ▁river ▁channel ▁migration ▁observed ▁in ▁the ▁L ena ▁River ▁of ▁Si ber ia ▁is ▁due ▁to ▁thermal ▁er os ion , ▁as ▁these ▁port ions ▁of ▁the ▁banks ▁are ▁composed ▁of ▁per ma f rost - cement ed ▁non - co hes ive ▁materials . ▁Much ▁of ▁this ▁er os ion ▁occurs ▁as ▁the ▁weak ened ▁banks ▁fail ▁in ▁large ▁sl umps . ▁Th erm al ▁er os ion ▁also ▁affect s ▁the ▁Ar ctic ▁coast , ▁where ▁wave ▁action ▁and ▁near - sh ore ▁temper atures ▁combine ▁to ▁under cut ▁per ma f rost ▁bl uff s ▁along ▁the ▁sh or eline ▁and ▁cause ▁them ▁to ▁fail . ▁Ann ual ▁er os ion ▁rates ▁along