log jam

File:DeadWoodGoodell Creek Log Jam.jpg, Washington]]

Log jams alter flow hydraulics by diverting flow towards the bed or banks, increasing flow resistance and creating upstream pools, diverting flow onto the floodplain and damming the channel, causing water to spill over the structure.{{cite journal|last1=Abbe|first1=T.B.|last2=Montgomery|first2=D.R.|title=Large woody debris jams, channel hydraulics and habitat formation in large rivers|journal=Regulated Rivers: Research & Management|date=1996|volume=12|issue=23|pages=201–221|doi=10.1002/(sici)1099-1646(199603)12:2/3<201::aid-rrr390>3.3.co;2-1}}{{cite journal|last1=Manners|first1=R.B.|last2=Doyle|first2=M.W.|last3=Small|first3=M.J.|title=Structure and hydraulics of natural woody debris jams|journal=Water Resources Research|date=2007|volume=43|issue=6|doi=10.1029/2006WR004910|bibcode=2007WRR....43.6432M|s2cid=129868907 |url=http://cdr.lib.unc.edu/downloads/n009w309g|access-date=2022-04-30|archive-date=2020-07-09|archive-url=https://web.archive.org/web/20200709224228/https://cdr.lib.unc.edu/downloads/n009w309g|url-status=live|doi-access=free}} These altered channel hydraulics change local patterns of erosion and deposition, which can create greater variety in local geomorphology and thus create provision and variety of habitat for instream living organisms.{{cite journal|last1=Gurnell|first1=A.M.|last2=Gregory|first2=K.J.|last3=Petts|first3=G.E.|title=The role of coarse woody debris in forest aquatic habitats: Implications for management|journal=Aquatic Conservation: Marine and Freshwater Ecosystems|date=1995|volume=5|issue=2|pages=143–166|doi=10.1002/aqc.3270050206|bibcode=1995ACMFE...5..143G }} The formation of a log jam against one bank typically concentrates flow in the wood-free portion of the channel, increasing velocity through this section and promoting scour of the riverbed. The formation of channel-spanning log jams can lead to the formation of an upstream pool, water spilling over the structure generating a "plunge pool" immediately downstream.{{cite journal|last1=Dixon|first1=S.J.|title=A dimensionless statistical analysis of logjam form and process|journal=Ecohydrology|volume=9|issue=6|pages=1117–1129|date=2015|doi=10.1002/eco.1710|s2cid=131127480 |url=http://eartharxiv.org/e4gt7/|access-date=2022-04-30|archive-date=2020-02-10|archive-url=https://web.archive.org/web/20200210175656/https://eartharxiv.org/e4gt7/|url-status=live}}

The hydraulic and geomorphological effects of log jams are highly dependent on the slope of the river (and thus the potential power of the stream); in steep channels, log jams tend to form channel-spanning steplike structures with an associated downstream scour pool,{{cite journal|last1=Curran|first1=J.C.|last2=Wohl|first2=E.E.|title=Large woody debris and flow resistance in step-pool channels, Cascade Range, Washington|journal=Geomorphology|date=2003|volume=51|issue=1–3|pages=141–157|doi=10.1016/S0169-555X(02)00333-1|bibcode=2003Geomo..51..141C}} whereas in large lowland rivers with low slopes, log jams tend to be partial structures primarily acting to deflect flow with minimal geomorphological change.{{cite journal|last1=Shields|first1=F.D.|last2=Gippel|first2=C.J.|title=Prediction of effects of woody debris removal on flow resistance|journal=Journal of Hydraulic Engineering|date=1995|volume=121|issue=4|pages=341–354|doi=10.1061/(ASCE)0733-9429(1995)121:4(341)}}

Log jams provide important fish habitat. The pools created and sediment deposited by formation of log jams create prime spawning grounds for many species of salmon. These pools also provide refuge for fish during low water levels when other parts of a stream may be nearly dry. Log jams can provide refuge, as velocity shelters, during high-flow periods.{{Citation needed|date=May 2021}}

It has been suggested that log jams are an aspect of trees acting as ecosystem engineers to alter river habitats to promote tree growth.{{cite journal|last1=Gurnell|first1=A.M.|title=Plants as river system engineers|journal=Earth Surface Processes and Landforms|date=2014|volume=39|issue=1|pages=4–25|doi=10.1002/esp.3397|bibcode=2014ESPL...39....4G|s2cid=55420478 |url=http://qmro.qmul.ac.uk/xmlui/handle/123456789/10726}} In dynamic braided rivers, such as the Tagliamento River in Italy, where the dominant tree species is the black poplar, fallen trees form log jams when they are deposited on bars; fine sediment is deposited around these log jams, and sprouting seedlings are able to stabilise braid bars and promote the formation of stable islands in the river. These stable islands are then prime areas for establishment of seedlings and further vegetation growth, which in turn can eventually provide more fallen trees to the river and thus form more log jams.{{cite journal|last1=Gurnell|first1=A.M.|last2=Petts|first2=G.E.|title=Trees as riparian engineers: The Tagliamento River, Italy|journal=Earth Surface Processes and Landforms|date=2006|volume=31|issue=12|pages=1558–1574|doi=10.1002/esp.1342|bibcode=2006ESPL...31.1558G|s2cid=129185856 }}

In large rivers in the Pacific Northwest of the United States, it has been shown there is a lifecycle of tree growth and river migration, with large trees falling into the channel as banks erode, then staying in place and acting as focal points for log jam formation. These log jams act as hard points, resisting further erosion and channel migration. The areas of floodplain behind these log jams then become stable enough for more large trees to grow, which can in turn become potential log jam anchor points in the future.{{cite journal|last1=Collins|first1=B.D.|last2=Montgomery|first2=D.R|last3=Fetherston|first3=K.L.|last4=Abbe|first4=T.B.|title=The floodplain large-wood cycle hypothesis: A mechanism for the physical and biotic structuring of temperate forested alluvial valleys in the North Pacific coastal ecoregion|journal=Geomorphology|volume=139-140|pages=460–470|doi=10.1016/j.geomorph.2011.11.011|year=2012|bibcode=2012Geomo.139..460C|doi-access=free}}

{{Wiktionary|logjam}}

"Logjam" or "log jam" can be used metaphorically to mean "deadlock" or "impasse." It can be used either more literally, to mean a physical impasse, or more metaphorically, to mean an impasse in a process due to differing opinions, legal or technical issues, etc.{{Cite web|url=https://www.merriam-webster.com/dictionary/logjam|title=Definition of LOGJAM|access-date=2022-04-30|archive-date=2022-03-23|archive-url=https://web.archive.org/web/20220323231553/https://www.merriam-webster.com/dictionary/logjam|url-status=live}} Here are two example sentences:

• "The presence of an ambulance on the side of the highway created a logjam of rubberneckers who just had to have a look." (more literal).
• "He was called in to try to break the logjam in the negotiations." (more metaphorical).

{{Commons category|Log jams}}

• Beaver dam, a wooden dam created by beavers
• Great Raft
• River morphology
• Stream restoration
• 1886 St. Croix River log jam

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Category:Aquatic ecology

Category:Geomorphology

Category:Rivers