Geomorphology of Channel Migration Zones And
Implications for Riparian Forest Management
Prepared by:
Matt O'Connor, PhD
O'Connor Environmental, Inc.
P.O. Box 794
Healdsburg, CA 95448
and
Greg Watson
Plum Creek Timber Company, L.P.
140 N. Russell
Missoula, MT 59801
June 1998
The concept of channel migration zones (CMZ’s) is a consequence of the fact that stream channels are dynamic features of the landscape that change position on valley floors under some circumstances. CMZ’s may be defined as terraces and/or floodplain areas adjacent to stream channels that have a high likelihood of being occupied by the stream channel at some time in the near future. The Washington Forest Practices Board (1995) defined the channel migration zone as “...the area that streams have recently occupied (in the last few years or less often decades), and would reasonably be expected to occupy again in the near future.” Lateral shifts of stream channels may occur suddenly as the result of flood flows and/or reduced channel capacity caused by sedimentation or debris jams, or gradually as a stream erodes the outside edge of meander bends.
Identification of CMZ’s and adoption of specialized management practices in these areas will reduce potential long-term adverse effects of riparian forestry on aquatic ecosystems. Existing regulations relating to forestry activities in riparian zones typically establish limits on harvest activities near stream channels based on the location of ordinary high water (OHW). These regulations are generally intended to provide for shading of the stream surface and recruitment of large woody debris (LWD) to stream channels. CMZ’s require specialized management because existing regulations may fail, in some areas, to provide the desired levels of shade or LWD recruitment following episodes of channel migration.
Lateral shifts in stream channels occur both suddenly and gradually. When a stream channel avulses, that is, when streamflow spills out of the banks of an existing channels, a new channel may be eroded in a short period. Channel avulsion typically occurs when the existing channel is incapable of carrying all of the water and sediment supplied to it.
During periods of high stream flow or floods, avulsions may occur at locations where channel morphology forces flow velocity to decrease and flow depth to increase. This often occurs at sharp bends in the channel. If the flow depth exceeds the height of the bank on the outside of the bend, water may spill over the bank. This water may then spread over a floodplain surface or, if the valley has a relatively steep slope and topographic features that concentrate the overbank flow, it may erode a new channel.
Channel avulsions are also caused by deposition of sediment and/or LWD that reduce channel capacity and increase flow depth (Keller and Swanson, 1979). Formation of large gravel bars or debris jams may also re-direct high velocity streamflow toward banks. When such flows encounter erodible banks and/or relatively low-lying terrace surfaces, a new channel may form. In some cases, the new channel will accommodate all of the streamflow and the former channel may be abandoned. In other cases, the new channel may carry excess flows only and the existing channel maintains its character. In still other cases, both new and existing channels may carry water under base flow conditions.
Gradual changes in channel position also occur, most often in low-gradient channels (approximately < 1% slope), as the outer bank of a meander bend is eroded during periods of routine (e.g. annual) peak flow. Bank materials for these low-gradient channels tend to be fine-grained, and are therefore susceptible to erosion. Avulsions may also occur in these types of streams, however, terrace surfaces adjoining such low-gradient streams tend to be nearly level and overbank flow is more likely to spread on a floodplain where there is insufficient slope to erode a new channel.
Gradual channel migration as conceived here occurs during most years at annual rates that are a fraction of the bankfull channel width. In contrast, channel migration by avulsion is conceived to occur sporadically and result in lateral shifts in channel position ranging in size from the bankfull channel width to the valley width. With respect to the former process, the rate of channel migration may be slow enough to allow harvest and regeneration of forest stands in riparian areas subject to channel migration. In the case of the latter process, channel migration is sudden and can occur anywhere within the CMZ. Consequently, if maintenance of aquatic ecosystem structure associated with channels in CMZ's is a priority; riparian forest conditions must be maintained in a state that would allow new channels to recruit significant quantities of LWD.
In order for channel migration to occur, the valley width must be sufficiently large to accommodate lateral shifts in position. Channels that are confined by valley walls are not prone to channel migration. The definition of a confined channel in common usage relates the bankfull channel width to the valley width. Rosgen (1994) defined the “flood prone width” as the width of a surface perpendicular to the trend of the valley or channel defined at an elevation twice that of the bankfull depth (corresponding to a flow with recurrence interval of about 1.5 to 2 years). The flood prone width corresponds to elevations of relatively frequent floods (< 50 yr recurrence interval). The ratio of flood prone width to bankfull width is the entrenchment ratio.
Entrenched channels have an entrenchment ratio (ER) £ 1.4, moderately entrenched channels have 1.4 < ER £ 2.2, and slightly entrenched channels have ER > 2.2 (Rosgen, 1994). Interpreted for purposes of predicting potential channel migration, slightly entrenched channels (Rosgen Types E, C, D and DA) have the greatest potential for channel migration, while moderately entrenched channels (Rosgen Type B) have modest potential. Entrenched channels (Rosgen Types A, F and G) have little or no potential for channel migration.
Using Rosgen’s system of channel classification, channels prone to channel migration might include Types E, C, D and DA, which are defined to have slopes < 2% (Rosgen 1994). Type B channels with some channel migration potential range in slope from 2% to 4%. Hence, channels with slope > 4% could be excluded on the basis of interpretation of Rosgen’s classification system.
An alternative to Rosgen is the channel classification system defined for watershed analysis by the State of Washington (Washington Forest Practices Board, 1995). In this system, confined channels are defined to be those in which the ratio of valley floor or floodplain width to channel width is < 2. Channels with moderate confinement have ratios between 2 and 4, and unconfined channels have values ratios > 4. The means to define the valley width in this system are not precisely defined, leading to a somewhat ambiguous classification. However, using this system, channels with valley width to channel width ratios > 2 could be expected to have potential for channel migration.
The Washington method classifies stream channels according to slope classes that roughly correspond to channel morphologic types defined by Montgomery and Buffington (1993). These slope classes are < 1%, 1-2%, 2-4%, 4-8%, 8-20% and > 20%. Subsequent research by Montgomery and Buffington (1997) redefined the slope classes corresponding to channel geomorphologic types as <1.5% (pool-riffle), 1.5-3% (plane-bed), 3-6.5% (step-pool), and >6.5% (cascade). Field observations by the author suggest that channel migration occurs in valleys as steep as the 8-20% slope class, typically in reaches where channel slope declines relative to areas upstream. However, as slope increases, valley confinement and channel entrenchment generally increase and thereby limit the horizontal range of migration. In other words, valley width is rarely >2 bankfull width in channels with slopes >8%. Field observations by the authors suggest that significant channel migration occurs infrequently where channel slope > 6%. On the basis of field experience and the Washington DNR watershed analysis classification system, CMZ’s would be limited to channels with slopes < 8% with moderately confined or unconfined boundaries.
For the purposes of investigating hypotheses proposed regarding CMZ processes, it is suggested that the entrenchment ratio definition of Rosgen (1994) be used in conjunction with slightly modified channel morphologic slope classes defined by Montgomery and Buffington (1993, 1997). Rosgen's entrenchment ratio is well-defined, and is a sensitive indicator of potential lateral flow dispersion that is required for channel migration. Montgomery and Buffington's channel classification system provides more complete (compared to Rosgen), description of channels steeper than 4%, and more explicitly integrates fluvial geomorphology and watershed process with channel classification. These slope classes are modified for classification of CMZ's (Table 2). Channels steeper than 8% are assumed to be sufficiently entrenched that the channel position cannot shift.
As noted earlier, channel migration by avulsion, and to some extent gradual migration by bank erosion, occur in response to deposition of coarse sediment or LWD in stream channels that locally reduce channel conveyance, forcing streamflow out of the existing channel. Channel migration in a particular stream reach subject to significant inputs of sediment or LWD will tend to occur if the channel is relatively unconfined (i.e. the ER is relatively large). Thus, in watersheds with higher rates of coarse sediment input, a higher incidence of channel migration might be expected in stream reaches with high ER's (Table 1). Reaches with high rates of LWD recruitment and/or LWD deposition and high ER's would also be expected to have higher incidence of channel migration. Channels with a large ER in watersheds with relatively low input rates for coarse sediment or LWD would be expected to have a significantly lower incidence of channel migration. Finally, channels with small ER's and low rates of LWD recruitment and low supply of coarse sediment would be expected to have no significant channel migration.
The geomorphic development of stream reaches with significant channel migration zones probably occurs over relatively short periods of geologic time (thousands of years). Channel migration processes require a relatively wide valley bottom with an elevation near that of the channel bed (i.e. a high ER). These conditions typically occur when the supply of coarse sediment is in balance with or greater than the stream's transport capacity for coarse sediment. When sediment supply exceeds transport capacity, stream channels aggrade (bed elevation increases), and tend to migrate laterally across the valley floor, depositing sediment on the floodplain. If sediment supply is interrupted, or transport capacity (streamflow) increases, stream channels begin to incise the valley deposits and become entrenched, and former floodplains may become terraces that are infrequently flooded or entirely isolated from the stream. Channel migration diminishes or ceases.
In the case of sediment supply ³ transport capacity, positive feedback tends to maintain conditions favorable to channel migration. Assuming a high ER, it is likely that a high water table will exist at least seasonally in the floodplain. The high water table increases the likelihood of significant blow down of trees. Recruitment of entire trees by this mechanism increases the potential for debris jam formation that induces channel avulsion. Channel avulsion also recruits LWD. High LWD concentration in stream channels increases channel roughness and decreases sediment transport capacity. Under these circumstances, it is possible that LWD recruitment to the channel may to some extent compensate for reductions in sediment supply and maintain conditions favorable to channel migration.
Table 1. Hypothesized likelihood of significant channel migration as a function of channel entrenchment (confinement) and supply of sediment and/or LWD.
|
Entrenchment Ratio |
Low Sediment/LWD Supply |
High Sediment/LWD Supply |
|
ER > 2.2 (unconfined) |
Moderate |
High |
|
1.4 < ER £ 2.2 (intermediate confinement) |
Low-Moderate |
Moderate-High |
|
ER £ 1.4 (confined) |
Low |
Low |
Historic changes in sediment supply and regional hydrology in many areas of the northwestern United States are related to the retreat of glaciers in the past 10,000 to 100,000 years. In most areas, stream channels have incised valley floor deposits and created one or more sets of terraces that are no longer subject to inundation. Thus, regional conditions suggest that incidence of channel migration should be diminishing. Areas where high LWD recruitment occurs may therefore be locations where channel migration processes are maintained, despite regional geologic trends that reduce potential for channel migration.
If this hypothesis is correct, decreases in long-term LWD recruitment to certain stream channels could induce essentially irreversible changes in channel morphology. Decreases in LWD recruitment could occur as the result of stand-replacing fires, logging, or other disturbance to riparian forest stands. In terms of gross channel morphology (and fish habitat), existing forced pool-riffle channels in reaches with active channel migration would be gradually transformed to entrenched plane-bed reaches. Changes in significant fish habitat variables likely would include increased particle size of stream bed sediment, reduced frequency and size of gravel bars, a decrease in the frequency and depth of pools, and decreased variation in stream velocity and depth.
Large scale production of coarse sediment by mass wasting, in some cases due to management, has been observed to induce channel aggradation and channel migration (O'Connor and Cundy 1993, O'Connor 1997). This occurs in geologic settings where channels are prone to migration processes, and have been subject to previous cycles of aggradation and degradation. In some areas where recent channel aggradation and migration has been observed, riparian forest stands in the channel migration zone had been converted to younger seral stands by prior timber harvest (O'Connor and Cundy 1993, O'Connor 1997). It is hypothesized that channel and aquatic habitat conditions in these areas would be significantly different had larger trees been present to be recruited when the stream channel began to aggrade. Large diameter, lengthy pieces of LWD with attached rootwads have the highest likelihood of remaining stable in a channel, and can establish stable nodes in the channel. Such "anchors" can promote the development of persistent pools, islands, and side channels that might otherwise be eroded or filled by coarse sediment in a stream reach where an episode of sediment-induced channel migration is occurring.
An hypothetical classification of channel migration potential by channel slope and entrenchment/confinement is presented in Table 2. This preliminary classification is based on prior observations of stream channels in the Pacific Northwest and professional judgment. It is intended as an initial hypothesis to be tested and revised through field observations. Figures 1 though 5 provide summary information and a sketch of representative channel conditions for the five cases in which significant channel migration is hypothesized in Table 2. Summary characteristics of the 5 proposed CMZ types are presented in Table 3 for ease of comparison.
In low-gradient channels (approximately < 1% slope), where channel migration is both gradual and sudden, stream channels are relatively sinuous, stream bends may have a short radius of curvature, and the pattern of channel migration can often be mapped from aerial photographs. Channels of this type are usually wide enough to be visible despite riparian canopy. In many cases, former channel positions may be determined on the basis of topography and/or vegetation, including oxbow lakes. The CMZ in this setting may often be defined in plan form as the area within the meander belt formed by the bends of the river or by the limits of the floodplain. In addition, field identification may benefit from relatively well-defined terraces that correspond to abandoned, inactive terraces within which a lower set of active terraces can be identified that correspond with the CMZ. It may be possible in some cases to quantify channel migration rates from aerial photo records for this channel type. If rates can be calculated, potential management responses can be made more objectively.
In steeper channels (approximately > 1% slope), channel migration is more likely to occur because of sudden channel avulsion. When the channel is wide enough to be visible despite the riparian canopy, it is often possible to observe the position of historic channels. In addition, vegetation types may aid in the identification of channel migration patterns. Mapping of historic channel shifts (where visible in photographs), may serve as the basis for CMZ delineation, however, field surveys may better define the specific circumstances under which channel avulsions occur and thereby narrow the spatial extent of the CMZ.
Stream channels narrower than about 30 to 40 feet typically can be difficult to observe in aerial photographs, depending on riparian forest conditions and the quality and scale of photography. In such areas, evidence of channel migration is difficult to obtain from aerial photographs. Field surveys would generally be required to locate riparian areas where channel migration processes are active. Characterization and mapping of areas of active channel migration is recommended in order to distinguish among areas of frequent and infrequent migration, which would likely have significant management implications.
Mapping the spatial and temporal frequency of channel avulsions could provide an objective basis for delineation of CMZ’s and development of management practices to maintain long-term riparian function. Frequency or rate of channel migration can possibly be accomplished through a combination of aerial photo mapping and field dendrochronology. Quantification of rates or frequency is desirable to provide more objective data for consideration of management responses.
Table 2. Classification of channel migration processes by stream channel slope and confinement classes and hypotheses regarding frequency and process.
|
Confinement or Entrenchment |
Low Gradient < 1.5 % Channel Slope |
Moderate Gradient 1 - 6 % Channel Slope |
High Gradient 4 - 8 % Channel Slope |
|
Confined/ Entrenched (ER £ 1.4) |
Uncommon channel type; no hypotheses regarding channel migration processes |
Channel migration unlikely |
Channel migration unlikely |
|
Moderately Confined/ Moderately Entrenched (1.4 < ER £ 2.2) |
Uncommon channel type; no hypotheses regarding channel migration processes |
CMZ C (3-6%) Channel migration by avulsion is not uncommon, but is likely to be spatially-discontinuous, depending on local variation in valley slope and width and disturbance regime |
CMZ E Channel migration by avulsion is uncommon, and is likely related to debris flows and torrents; may be locally significant depending on local variation in valley slope and width and disturbance regime |
|
Unconfined or Slightly Entrenched (ER > 2.2) |
CMZ A (<1.5%) Channel migration by gradual erosion of meander bends is common; avulsions may also occur. Areas of potential migration are spatially continuous and include much or all of the floodplain. |
CMZ B (1-3.5%) Channel migration by avulsion may be common. Areas of potential migration are spatially discontinuous and include much or all of the floodplain. |
CMZ D Channel migration by avulsion may be common, and is likely to be related to alluvial fan and debris flow fan processes. Process may be spatially discontinuous continuous, or localized, depending on fan-building processes. |
Table 3. Summary characteristics of hypothesized channel migration zone (CMZ) types.
|
CMZ CHARACTERISTIC |
TYPE A |
TYPE B |
TYPE C |
TYPE D |
TYPE E |
|
Diagnostic Characteristics |
|
||||
|
Channel Migration Processes |
Bank erosion (primary) and Avulsion (secondary) |
Avulsion (primary) and bank erosion (secondary) |
Avulsion |
Fan-related (alluvial & debris flow fans) |
Avulsion (debris flow & debris torrent) |
|
Entrenchment Ratio |
ER > 2.2 |
ER > 1.4 |
ER > 1.4 |
ER > 2.2 |
1.4 < ER £ 2.2 |
|
Slope |
< 1.5 % |
1-3.5 % |
3-6 % |
4-8 % |
4-8 % |
|
Channel Morphology |
Pool-riffle, plane-bed |
Forced pool-riffle, plane bed, step-pool |
Step pool, forced pool-riffle, plane bed |
Step-pool, cascade, plane bed |
Step-pool, cascade, plane bed |
|
Supplementary Descriptive Characteristics |
|
||||
|
Longitudinal Distribution of CMZ |
Continuous |
Continuous or Discontinuous |
Discontinuous |
Continuous or Discontinuous |
Discontinuous |
|
Sinuosity |
High |
Moderate |
Moderate |
Low |
Low |
|
Channel Bed Substrate |
Silt, Sand, Gravel, Cobble |
Gravel, Cobble |
Gravel, Cobble, Boulder |
Cobble, Boulder |
Cobble, Boulder |
|
Bank Material |
Same as be |
Same as bed or coarser |
Same as bed or coarser |
Typically same as bed, possibly coarser |
Typically coarser than bed, possibly same |
|
Relative Landscape Frequency |
Common |
Common |
Uncommon |
Rare (active fans) |
Rare (depends on landslide type & frequency) |
|
Management Sensitivity |
|
||||
|
Channel Migration Potential (1) |
High |
High |
High |
Moderate |
Low-Moderate |
|
CMZ Sensitivity to LWD (2) |
Moderate |
High |
High |
Moderate |
High |
|
CMZ Sensitivity to Coarse Sediment |
High |
High |
High |
High |
Moderate |
Notes
1. Channel migration potential is evaluated relative to other CMZ types. The actual occurrence of channel migration is thought to be a function of both local and upstream disturbance (e.g. Table 1), and variation in local channel slope and confinement within a reach of the given type.
2. The influence of LWD on channel migration processes is affected by channel width. As bankfull channel width increases, a smaller proportion of LWD is likely to have an effect on channel morphology because LWD pieces that are shorter than the bankfull width will tend to be transported downstream. In Washington and Oregon, LWD abundance in stream channels with bankfull widths between 30 and 50 feet decreases significantly compared to narrower channels (e.g. Bilby and Ward, 1991).
The preceding sections described CMZ processes and identification. This section discusses potential response to CMZ delineation in the context of riparian forest management. Riparian forests generally have significant existing regulatory protection (e.g. Montana Streamside Management Zone Law). The central objective of the following discussion is to consider whether existing riparian regulations are sufficient for CMZ's, and if not, what actions should be considered to enhance the protection of aquatic ecosystem function in CMZ's. In general, when existing riparian management zones have a width on each stream bank less than the bankfull channel width, it is much more likely that existing leave tree requirements will be inadequate. If the width of the existing riparian management zone on each bank is a multiple (e.g. 3 or 4) of the bankfull channel width, it is much more likely that existing leave tree requirements will be adequate.
An identified CMZ may fall entirely within the limits of existing regulatory streamside or riparian management zones. In these cases, the primary concern is whether existing regulatory requirements for leave trees are sufficient to maintain aquatic ecosystem function in the CMZ. The number, size and location of leave trees should be considered in relation to the number and size of LWD pieces functioning in the CMZ. The management goal should be to ensure that if a channel shift occurs, the number, size and species of LWD pieces recruited to the new channel is comparable to that in existing channels in adequately-functioning comparable channels in comparable CMZ's. Potential differences in the importance of LWD in different types of CMZ's (see Table 3), might also serve as a criteria. In addition, estimates of frequency and spatial distribution of channel migration events (avulsions) or rate of bank erosion (gradual migration) could be relevant factors in determining appropriate criteria for leave trees.
An identified CMZ may extend beyond the limits of existing regulatory streamside or riparian management zones. In these cases, the main concerns are the risk of channel migration beyond the existing leave tree zone and the adequacy of existing leave tree requirements for maintenance of aquatic ecosystem function in the CMZ. At a minimum, extending the regulatory requirements for leave trees in the riparian management zone to include the CMZ should be considered. Estimates of the frequency and spatial distribution of channel migration events (avulsions) or rate of bank erosion (gradual migration) that could lead to channel migration beyond the boundary of the existing riparian management zone could be relevant in determining appropriate criteria for leave trees in the CMZ outside the riparian management zone. The number, size and location of leave trees should be considered in relation to the number and size of LWD pieces functioning in the CMZ. The management goal should be to ensure that if a channel shift occurs, the number, size and species of LWD pieces recruited to the new channel is comparable to that in existing channels in adequately functioning comparable channels in comparable CMZ's. Potential differences in the importance of LWD in different types of CMZ's (see Table 3), might also serve as a criteria.
Further research is needed to test the applicability of the proposed CMZ classification system. This should include a literature review, but emphasis should be on field identification and mapping. An investigation of the relationship between LWD abundance, function and size in relation to stream size and CMZ type would help provide criteria for leave trees. Data on the frequency and rate of channel migration by both avulsion and bank erosion would provide relevant perspective on the design of management strategies for maintaining aquatic ecosystem function in CMZ's.
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Keller, E.A. and Swanson, F. J., 1979. Effects of large organic debris on channel form and fluvial processes. Earth Surface Processes, Vol. 4, pp. 361-380.
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Montgomery, D.R. and Buffington, J.M. 1997. Channel-reach morphology in mountain drainage basins. GSA Bulletin 109(5):596-611.
O'Connor M.D. and Cundy, T.W. 1993. North Fork Calawah River Watershed Condition Survey, Part I: Landslide Inventory and Geomorphic Analysis of Mass Erosion; Part II: Channel Condition and Cumulative Effects of Mass Wasting in Headwater Tributaries". Unpublished technical reports prepared under contract to U.S.D.A. Forest Service, Olympic National Forest. Part I: 17 pp. plus 12 figures; Part II: 33 pp., 39 plates.
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