12 Drainage Basins Stream order (Strahler) Means of classifying basin size1st order = no tributaries2nd = two 1st3rd = two 2nd
13 14 m across3200 m acrossLandscapes consist of ridge and valley topography at all scales, but only finest scale reveals the actual valley network and defines the transition between hillslopes and valleyfractals exhibit self-similarity; that is similar structure is observed at many scales. With self-similarity, the value of a property depends on the resolution of observationMontgomery and Dietrich, 1992, Science
14 Channel networks are of finite extent. bbdChannel networks are of finite extent.The spacing of the finest-scale valleys depends on the competition of valley cutting and hillslope eroding processes.Fractal analysis breaks down at the channel-hillslope transition.River basin comprised of network and hillslopes. Hillslopes control production of runoff, sediment, and wood. Network routes these through basin.Spacing or drainage density is another way of saying how well dissected is the landscape? Dissection limited by threshold of channelization.
15 Limitations of Horton-Strahler ordering Order does not express the intuitive ‘size’ of a catchment very wellThese look very similar in size, but the left hand net is of order 2, and the right hand net or order 3Both of these are of order 2, but one looks much ‘bigger’ than the other!
16 Magnitude: an alternative approach (Shreve) The left hand net has a magnitude of 9, and the right hand has a magnitude of 2Both of these very similar networks have the same magnitude of 4Magnitude may give a better idea of the size of the networkShreve explored all the possible network topologies for a given magnitude
17 Magnitude and stream order Magnitude of a basin is the number of first order or “exterior links” in a catchment. Magnitude correctly emphasizes identifying where the channel begins. Stream order is commonly done on nearly arbitrary network scales, and therefore means little. “Horton’s laws”, which are derived from analysis of stream orders, have no physical meaning.M = 9N= 2M-1N= number of links (exterior + interior)For channel head theory, see:Istanbulluoglu et al. JGR 2005 for gully head theoryTucker and Bras, 1998, WRR for theory to landscape evolution
19 Watershed Networks Watershed network comprised of 7,10014,20021,30028,4003,550MetersLegendnednet<all other values>Order1234Logan River Network and Watershedsdelineated by TauDEMWatershed network comprised ofheadwater and network systemsFirst and second order streams often comprise 70% of the stream network (Benda et al, 1992)High ecological valueStream networks defined by:Nat’l Hydrography Dataset (1:100,00)Terrain analysis(area, area-slope; area-length thresholds)
20 Effects of low order channels on downstream reaches in the network Synchronous (or asynchronous) inflows of water, sediment, nutrients, and organic matter create a variety of channel conditions and biological assemblagesConnectivity of headwater systems to downstream reaches affects the cumulative and dispersed nature of material transport processesGomi, et al, Understanding processes and downstream linkages of headwater systems, BioScience, Oct. 2002, vol. 52, no. 10
21 Watershed Delineation by Hand Digitizing Watershed divideDrainage directionOutletArcHydro Page 57
22 River networks and channel morphology Where do channels begin?Channel network structure
23 Channel InitiationChannel head: the upstream limit of concentrated water flow between banks (Dietrich and Dunne, 1993)a major boundary between hillslopes and channels“pivot point” in sediment transport between diffusive process and incisive processChannel initiation requires runoffChannel initiation occurs by:saturated overland flowseepage erosionshallow landsliding
24 Channel Head Location and Topography Montgomery and Dietrich, 1989
25 Channel head: threshold transition between hillslope and channel processes Interplay of channel and hillslope evolution. Channel initiation = f( exposed lithology, vegetation cover, prevailing erosional mechanism) Whatever that might be: SOF, OFchannel head location influenced by area and slope
28 Channel Initiation and Basin Morphometry Process model for channel initiation by shallow landslidingconvergent topography causes colluvium eroded from adjacent hillslopes to accumulateat critical threshold, landsliding occurs exposing underlying materialerosion of underlying material by saturation overland flow initiates channelChannel heads controlled by hillslope process rather than network extensionInverse of source basin length ~ drainage density
29 Valleys Hillslopes Dietrich et al., 2003, PIG channel heads assoc. with a change in sediment transport processes at a critical contributing area. Hillslope sed trans: slope-dependent. Ch. sed trans: slope and discharge-dependentHillslopesDietrich et al., 2003, PIG
31 Coos Bay, OR Dietrich & Perron, in press,Nature inflection point is the handoff between landscape disturbances, typically thought of as mass wasting dominated regime to fluvial dominated regimeLocation of inflection point reflects general environmental controls on ch. initiation (cli, veg)Dietrich & Perron, in press,Nature
32 Montgomery and Dietrich, 1992, Science N. CaliforniaOregon Coast Rangechanneledchannel headunchanneledS. CaliforniaSummarychannel heads are defined through a topographically defined ( CA and S) threshold between channeled and unchanneled regions of the landscapethreshold behaviorMontgomery and Dietrich, 1992, Science
33 a= roughness and transport parameters T= transmissivityq= effective rainfallM=slopeF = friction angleq= slope anglea= roughness and transport parametersprevailing erosional mechanism alluded to earlierExample for humid, soil mantled landscapeDietrich et al Geology
34 unchanneled = hillslope Application of slope- area channel threshold to a digital terrain modelchanneledunchanneled = hillslopea/b a proxy for contributing area. Mapped channel versus applying slope-area threshold to a DEM For the threshold, no channels observed below this rangenetwork topology affects debris flow frequency and slope-area relationtransitionObserved channel in the fieldMontgomery and Dietrich, 1992, Science
35 SummaryChannel heads typically occur at threshold changes in dominant transport process, and depend both on drainage area and local slope.Channel network structure imparts a transport distance structure (width function) that influences sediment routing.