Spatial and temporal geomorphic variability in fluvial systems: A case study from the Conejos River, Colorado
Shortcut URL: https://serc.carleton.edu/60212
Location
Continent: North America
Country: USA
State/Province:Colorado
City/Town: Platoro
UTM coordinates and datum: none
Setting
Climate Setting: Semi-Arid
Tectonic setting: Continental Collision Margin
Type: Stratigraphy, Chronology
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Description
Effective and sustainable river management requires an understanding of how fluvial processes vary both spatially and temporally (e.g. Wohl et al., 2005). This is important because rivers are capable of alternating between widely different process regimes (e.g. vertical incision and aggradation) even at short timescales. Currently, however, management practices are often implemented with little recognition of the natural variability of fluvial processes.
This case study examines the post-glacial temporal and spatial variability for the Conejos River, southern Colorado. We investigated the relationship between the formation of fluvial terraces and sediment supplied from hillslopes and tributary streams. The study area includes three sub-reaches; Platoro, Lake Fork and South Fork (Fig. 1). These sub-reaches are characterized by broad glacial valleys, separated by steeper canyon reaches, in which a range of alluvial, colluvial and glacial deposits and landforms are evident (Figs. 2 & 3).
Temporal variability
A radiocarbon (14C) chronology was developed from charcoal buried in alluvial deposits and compared to a local paleoclimate record (Johnson, 2010) (Fig. 4). Since glaciers retreated (~11 ka), there have been three episodes of terrace strath formation (8.9 – 7.6 ka, 5.5 ka and 3.5 – 1.1 ka) separated by periods of vertical incision. Streams typically alter from vertical to lateral incision (i.e. strath cutting) due to increases in sediment supply (e.g. Hancock and Anderson, 2002). We have evidence for a range of processes, operating at different times, which resulted in increased sediment supply in the Conejos River Valley. Therefore, it is important to investigate potential causes of terrace formation by utilizing multiple working hypotheses. The most likely hypotheses for increases in sediment supply in this area are due to 1) climatic cooling, 2) increased frequency of climate change and 3) increased fire frequency.
Fluvial deposition at 8.9 ka (Qt1) correlates with a period of cooler climatic conditions (Fig. 4). Colder climates likely created longer-lived snow fields, which contributed to extreme runoff events in tributary streams during summer rains. Melting snow fields also leave vegetation-bare earth particularly susceptible to erosion during the late spring and summer. These conditions favored the movement of sediment to the valley bottom, which resulted in a switch from vertical incision to lateral strath carving at this time.
Alluvial deposition at 5.5 ka (Qt2 and Qaf2) correlates to 1) a brief period of local climatic cooling and 2) to a change in the frequency of climate oscillations (Fig. 4). Changes in the frequency of climatic changes have been attributed to strengthened El Niño-Southern Oscillation (ENSO) cycles, which likely resulted in high winter snow packs and intense rain-on-snow events during El Niño years. However, regional paleoclimate studies document warm, dry conditions from 6 – 4 ka, with peak fire event frequencies. Therefore, an alternative hypothesis is that an increase in forest fires resulted in sedimentation at 5.5 ka, i.e. fire-related sediments were transported to the valley floor during intense storm events that typically occur during warm, drought prone periods.
Qt3 is a fill terrace, which indicates that sediment input during 2.2 – 1.1 ka was substantially higher relative to other periods of sediment deposition and resulted in significant aggradation in the valley bottom. There is no clear cooling trend at this time, however, it is possible that sediment supply increased during the cooling limb of an earlier climatic cycle (3.5 – 3.0 ka) (Fig. 4). Also, the frequency of climate fluctuations increases again after 3.5 ka suggesting that aggradation may have begun at this time due to further ENSO strengthening. Additionally, regional climate studies indicate overall drier climates post 3.5 ka. We suggest that rainfall events were strong enough to erode sediments, but did not supply enough water to the main channel to remove sediment deposited in the valley bottom. Furthermore, drier climates likely resulted in more fire-related sediments. This hypothesis is supported by the presence of charcoal layers in Qaf3 deposits and evidence for many forest fires in the region between 2.8 – 1.8 ka.
The latest aggradational phase (Qt3) was punctuated by incision post 1 ka. The local climate record is difficult to interpret during this period with evidence of warming and cooling exhibited by different pollen records (Fig. 4). A radiocarbon date from a floodplain deposit suggests that the modern floodplain was stable at 150 ybp. It is therefore possible that incision into Qt3 deposits occurred during the globally documented Medieval Warm Period (~950 – 750 ybp), with incision being halted by increased sediment supply during the most recent cold period of the Little Ice Age (~400 – 100 ybp).
Spatial variability
Stream and landform morphology also varies longitudinally due to the influence of remnant glacial topography (Fig. 5). During glaciation, increased ice input from tributary valley glaciers resulted in deep glacial incision at tributary confluences. In contrast to the longitudinal profiles of rivers, glaciated valleys typically have wide, low gradient floors punctuated by multiple steps and overdeepenings tens to hundreds of meters deep. These steps were in turn filled with glacial outwash or till during glacial retreat. In these locations, later terrace straths were cut into glacial deposits rather than bedrock (i.e. they are fill-cut terraces rather than strath terraces).
Conclusion
In the Conejos River watershed, several restoration projects have been suggested to combat severe streambank erosion. However, these proposals do not take into account the natural range of variability for this fluvial system. As this case study highlights, the Conejos River has fluctuated between episodes of strath formation, valley aggradation, and vertical incision since deglaciation. This variability raises questions regarding the 'natural' conditions of the system. Additionally, stream processes are influenced by the topography left behind by retreating glaciers where steps at tributary confluences have resulted in fluvial incision into glacial deposits rather than bedrock. This is important because glacial deposits are more easily eroded than bedrock. Overall, evaluating temporal and spatial variability is important in order to produce sustainable and effective management plans.
This case study examines the post-glacial temporal and spatial variability for the Conejos River, southern Colorado. We investigated the relationship between the formation of fluvial terraces and sediment supplied from hillslopes and tributary streams. The study area includes three sub-reaches; Platoro, Lake Fork and South Fork (Fig. 1). These sub-reaches are characterized by broad glacial valleys, separated by steeper canyon reaches, in which a range of alluvial, colluvial and glacial deposits and landforms are evident (Figs. 2 & 3).
Temporal variability
A radiocarbon (14C) chronology was developed from charcoal buried in alluvial deposits and compared to a local paleoclimate record (Johnson, 2010) (Fig. 4). Since glaciers retreated (~11 ka), there have been three episodes of terrace strath formation (8.9 – 7.6 ka, 5.5 ka and 3.5 – 1.1 ka) separated by periods of vertical incision. Streams typically alter from vertical to lateral incision (i.e. strath cutting) due to increases in sediment supply (e.g. Hancock and Anderson, 2002). We have evidence for a range of processes, operating at different times, which resulted in increased sediment supply in the Conejos River Valley. Therefore, it is important to investigate potential causes of terrace formation by utilizing multiple working hypotheses. The most likely hypotheses for increases in sediment supply in this area are due to 1) climatic cooling, 2) increased frequency of climate change and 3) increased fire frequency.
Fluvial deposition at 8.9 ka (Qt1) correlates with a period of cooler climatic conditions (Fig. 4). Colder climates likely created longer-lived snow fields, which contributed to extreme runoff events in tributary streams during summer rains. Melting snow fields also leave vegetation-bare earth particularly susceptible to erosion during the late spring and summer. These conditions favored the movement of sediment to the valley bottom, which resulted in a switch from vertical incision to lateral strath carving at this time.
Alluvial deposition at 5.5 ka (Qt2 and Qaf2) correlates to 1) a brief period of local climatic cooling and 2) to a change in the frequency of climate oscillations (Fig. 4). Changes in the frequency of climatic changes have been attributed to strengthened El Niño-Southern Oscillation (ENSO) cycles, which likely resulted in high winter snow packs and intense rain-on-snow events during El Niño years. However, regional paleoclimate studies document warm, dry conditions from 6 – 4 ka, with peak fire event frequencies. Therefore, an alternative hypothesis is that an increase in forest fires resulted in sedimentation at 5.5 ka, i.e. fire-related sediments were transported to the valley floor during intense storm events that typically occur during warm, drought prone periods.
Qt3 is a fill terrace, which indicates that sediment input during 2.2 – 1.1 ka was substantially higher relative to other periods of sediment deposition and resulted in significant aggradation in the valley bottom. There is no clear cooling trend at this time, however, it is possible that sediment supply increased during the cooling limb of an earlier climatic cycle (3.5 – 3.0 ka) (Fig. 4). Also, the frequency of climate fluctuations increases again after 3.5 ka suggesting that aggradation may have begun at this time due to further ENSO strengthening. Additionally, regional climate studies indicate overall drier climates post 3.5 ka. We suggest that rainfall events were strong enough to erode sediments, but did not supply enough water to the main channel to remove sediment deposited in the valley bottom. Furthermore, drier climates likely resulted in more fire-related sediments. This hypothesis is supported by the presence of charcoal layers in Qaf3 deposits and evidence for many forest fires in the region between 2.8 – 1.8 ka.
The latest aggradational phase (Qt3) was punctuated by incision post 1 ka. The local climate record is difficult to interpret during this period with evidence of warming and cooling exhibited by different pollen records (Fig. 4). A radiocarbon date from a floodplain deposit suggests that the modern floodplain was stable at 150 ybp. It is therefore possible that incision into Qt3 deposits occurred during the globally documented Medieval Warm Period (~950 – 750 ybp), with incision being halted by increased sediment supply during the most recent cold period of the Little Ice Age (~400 – 100 ybp).
Spatial variability
Stream and landform morphology also varies longitudinally due to the influence of remnant glacial topography (Fig. 5). During glaciation, increased ice input from tributary valley glaciers resulted in deep glacial incision at tributary confluences. In contrast to the longitudinal profiles of rivers, glaciated valleys typically have wide, low gradient floors punctuated by multiple steps and overdeepenings tens to hundreds of meters deep. These steps were in turn filled with glacial outwash or till during glacial retreat. In these locations, later terrace straths were cut into glacial deposits rather than bedrock (i.e. they are fill-cut terraces rather than strath terraces).
Conclusion
In the Conejos River watershed, several restoration projects have been suggested to combat severe streambank erosion. However, these proposals do not take into account the natural range of variability for this fluvial system. As this case study highlights, the Conejos River has fluctuated between episodes of strath formation, valley aggradation, and vertical incision since deglaciation. This variability raises questions regarding the 'natural' conditions of the system. Additionally, stream processes are influenced by the topography left behind by retreating glaciers where steps at tributary confluences have resulted in fluvial incision into glacial deposits rather than bedrock. This is important because glacial deposits are more easily eroded than bedrock. Overall, evaluating temporal and spatial variability is important in order to produce sustainable and effective management plans.
Associated References
- Layzell AL, Eppes MC, Johnson BG, Diemer JA. In review. Post-glacial range of variability in the Conejos River Valley, Southern Colorado, USA: Fluvial response to climate change and sediment supply. Earth Surface Processes & Landforms.
- Hancock GS, Anderson RS. 2002. Numerical modeling of fluvial strath-terrace formations in response to oscillating climate. Geological Society of America Bulletin 114: 1131-1142. DOI: 10.1130/0016-7606(2002)114<1131:NMOFST>2.0.CO;2
- Johnson BG. 2010. Alpine and sub-alpine landscape response to post-glacial climate change in the San Juan Mountains: A comparison of new landscape and climate records. PhD Dissertation, University of North Carolina at Charlotte.
- Wohl E, Angermeier PL, Bledsoe B, Kondolf GM, MacDonnell L, Merritt DM, Palmer MA, Poff L, Tarboton D. 2005. River restoration. Water Resources Research 41 W10301. DOI:10.1029/2005WR003985