USGS Scientific Investigations Report 2010–5016 Development of an Environmental Flow Framework for the McKenzie River Basin, Oregon

Table 21. Streamflow and physical habitat relationship summary for the upper, middle, and lower McKenzie River basins, Oregon.

Conceptual model of relationship between streamflow and physical habitat for McKenzie River
Habitat type	Primary constraints for different McKenzie River basin sections	Effect of different magnitude flow events on physical habitat
Habitat type		Drought / Low flow (channel bottom)	High flow pulse (Up to bankfull)	Small flood (Overbank, 2–10 year recurrence interval)	Large flood (Floodplain maintenance, >10-year recurrence interval)
Pool complexity	Upper basin (Reaches 1-2): Most pools currently formed at bedrock outcrops; cover is mainly provided by steep banks and depth.	No change	Maintain existing pools by flushing fines and smaller gravels.	Maintain existing pools by flushing fines, smaller gravels and deepening pools.	Channel shifting could lead to creation of new pools.
	Middle basin (Reaches 3-8): Most pools formed at bedrock outcrops, and at bends in channel; cover mainly provided by steep banks and depth.		Possibly enhance pool complexity through local bank erosion and wood recruitment.	Possibly create new scour pools along alluvial reaches where deposition leads to bar/island growth.	Scour and deepen existing pools; possibly fill in other pools.
	Lower basin (Reaches 9-12): Presently, there are few large wood jams and islands that historically supported scour pools; most pools now formed at bedrock outcrops or by scour adjacent to revetment; limited cover.			Enhance pool complexity by recruiting large wood via bank erosion, and by deepening existing pools.	New pools could be created where new bars/islands are formed.
					Pool complexity could be enhanced by deepening and through recruitment of large wood from bank erosion.
Off-channel habitat	Upper Basin (Reaches 1-2): Few existing secondary channel features due to steep, confined channel and floodplain. Typical flood flows probably insufficient to carve new side-channels except near Rainbow.	Little to no change	Inundate low-lying features and recharge groundwater.	Inundate secondary channels along floodplain.	Carve and erode new secondary channels along the tops of bars, islands and possibly on the floodplain.
	Middle Basin (Reaches 3-8): Sparse secondary features due to narrow, confined floodplain. Best opportunities for off-channel habitat are between Finn Rock and Blue River.		Flush fine sediments, possibly disrupt and remove young vegetation.	Scour and maintain low elevation channels in active channel.	Possibly trigger avulsions that would result in abandonment of existing main channel.
	Lower Basin (Reaches 9-12): Historically supported abundant off-channel habitat; currently limited by lack of peak flows, revetments and sediment supply.			Possibly carve and create new lower-elevation channel along erodible areas of active channel.	Scour and erode existing secondary channel features.
					Remove vegetation from many surfaces in active channel, making it easier for smaller floods to re-mobilize these surfaces.
Gravel bars and spawning gravels	Upper Basin (Reaches 1-2): Low sensitivity to discharge regime. Sparse gravel bars due to naturally low sediment supply which is further reduced by dams. High transport capacity results in limited areas where gravel can deposit. High flow pulses unlikely to have large impact unless accompanied by increase in supply from tributaries and/or gravel augmentation.	Little to no change for gravel bars.	Local bank erosion may release some gravels and large wood that could enhance bar formation.	More significant re-mobilization of existing bars.	Possibly mobilize coarse in-channel sediment and disrupt the armor layer.
	Middle Basin (Reaches 3-8): Low sensitivity to discharge regime. Sediment supply substantially reduced due to dams on South Fork and Blue River. High transport capacity, with few areas where bars can form. High flow pulses unlikely to have large impact unless accompanied by increase in supply from tributaries and/or gravel augmentation.	May flush fine sediments from spawning gravels.	May reduce vegetation on low bar surfaces; particularly important for Lower Basin.	More substantial bank erosion, particularly along lower reaches could recruit bed-material and wood.	Substantial bank erosion into floodplain deposits could introduce large wood and gravels, enhancing bar forms.
	Lower Basin (Reaches 9-12): High sensitivity to discharge regime. Gravel bars limited by reduced sediment supply (due to dams on South Fork and Blue River). Flow reduction has resulted in vegetation colonization, reducing the area of active gravel bars. Local recruitment of gravel via bank erosion is limited by revetments and reduction of peak flows.		Re-mobilize existing unvegetated bars; mobilize and transport spawning size-gravels.	More significant disruption of vegetation on bars, allowing for greater degree of re-mobilization.	Disrupt and re-mobiilze densely vegetated bars at numerous sites that were historically bare.
					Potentially trigger large-scale changes that would enhance reach-wide bar formation (e.g., formation of large new bars, which in turn trigger bank attack, adding to bar growth).
Floodplain formation	Upper Basin (Reaches 1-2): Floodplain is narrow and confined; only limited areas near Rainbow where more complex floodplain processes can occur.	Little to no change	Floodplain margins could be modified by bank erosion.	Inundation and scour along low-lying floodplain surfaces could increase patch heterogeneity.	Widespread inundation and scour along floodplain channels could substantially increase patch heterogeneity.
	Middle Basin (Reaches 3-8): Confined, narrow floodplain along most of Reaches 6-8 due to valley physiography. Peak flow reduction minimizes floodplain formation along historically complex floodplain areas above Finn Rock.			More significant bank erosion could ‘recycle’ floodplain by recruiting sediment and wood that could be deposited elsewhere.	Possible avulsions and carving of new floodplain channels.
	Lower Basin (Reaches 9-12): Floodplain formation processes limited by peak flow reduction, revetments and decreased supply of sediment and large wood. Depending on sediment transport, incision could be locally exacerbated by high flows.				Overbank deposition would enhance formation of incipient floodplain
					Potential for substantial erosion into floodplain margins which could recruit bed material and wood that can be deposited elsewhere, forming bars that may eventually evolve into floodplain.