Contrasting climate controls on the hydrology of the mountainous Cauca River and its associated sedimentary basin: Implications for interpreting the sedimentary record
Introduction
Rivers route sediment sourced from eroding uplands toward deposition in sedimentary basins (Allen, 2008). The flux of sediment through a river is modulated by a complex interaction of tectonic (e.g., rock uplift or subsidence) and climatic processes (e.g., variability in precipitation) as it propagates downstream to the sedimentary basin (Jerolmack and Paola, 2010; Romans et al., 2016). Any potential signal generated by climatic or tectonic activity in the hinterland is often distorted en route to the basin, creating a challenge for geologists attempting to reconstruct the magnitudes and rates of processes that affected the production, transport and deposition of sediments in a basin (Leeder, 1997). Examining how signals are propagated through modern fluvial systems offers insights crucial to interpreting the geologic record.
In mountainous rivers, sediment production and transport may be dominated by factors such as the rate of rock uplift (e.g., Hovius, 1998), relief and lithology (Aalto et al., 2006; Carretier et al., 2018), patterns in precipitation/runoff (Pratt et al., 2002; Restrepo et al., 2006a), anthropogenic factors (Restrepo and Escobar, 2016) or a combination of these processes. High and low-frequency climatic oscillations such as the movement of the Intertropical Convergence Zone (ITCZ), El Niño Southern Oscillation (ENSO), the Atlantic Meridional Oscillation (AMO), or the Pacific Decadal Oscillation (PDO), might play a significant role in modulating runoff and sediment transport in catchments of the American tropics at a regional scale (Morera et al., 2017; Poveda, 2004; Restrepo et al., 2019). For example, mountainous rivers draining the eastern flank of the Andes into the Amazon Basin display large variations in runoff and specific stream power (erosion) during different ENSO phases (Bookhagen and Strecker, 2010). The mountainous rivers draining the Sierra Nevada de Santa Marta in northern South America show a correlation between runoff and lower frequency oscillations such as the Atlantic Meridional Oscillation (AMO), Pacific Decadal Oscillation (PDO) and the Tropical North Atlantic Index (TNA) (Restrepo et al., 2019).
In lowland sedimentary basins, climate driven variability in river discharge can trigger episodic sediment deposition via river avulsion or overbank flooding. High-runoff events during the negative phase of ENSO (La Niña) generate floodplain sedimentation in the Andean foreland (Aalto et al., 2003) and river avulsion in the Magdalena River in the Mojana Basin (Morón et al., 2017). It is unclear if the signal in runoff associated with ENSO originates from precipitation in the upland mountainous catchment or locally from precipitation above the lowland sedimentary basin. From a geological perspective, sedimentary rocks might preserve geochemical or sedimentological information about climatic or tectonic processes generated in the upland region, tens to hundreds of kilometers away from the site of deposition, or locally from processes occurring within the lowland sedimentary basins.
The Cauca River of the tropical Northern Andes and the adjacent lowland Mojana sedimentary basin (Fig. 1) represents an ideal setting to study the interconnection between tectonic (e.g., rock uplift) and climatic elements (e.g., ITCZ, ENSO, AMO, PDO) on water and sediment discharge, and how these compare to climatic factors that drive variability in deposition in the lowland sedimentary basin, through avulsion events and seasonal wetland flooding. The Cauca River traverses a longitudinal valley bounded by tectonically active mountain ranges where it experiences gradients in uplift and subsidence as well as strong spatial variations in climate that affect runoff and sediment generation to the lowland sedimentary basin. On the other hand, the Mojana Basin is an actively subsiding sedimentary basin where active sediment accumulation is occurring through processes such as river avulsion, channel bifurcation, floodplain deposition and seasonal wetland sedimentation (Morón et al., 2017; Smith, 1986; Weissmann et al., 2015).
In this paper we studied the hydrology and geomorphology of the Cauca River by integrating satellite imagery, rainfall and historic hydroclimate data. First, we analyze the periodicity of runoff and precipitation data for the catchment upstream of the alluvial transition along with wetland water level and precipitation for the sedimentary basin in order to infer what climate phenomena drive inter and intra-annual variability in each of these regions (Fig. 1). Then, we focus on determining what controls the spatial and temporal patterns in erosion in the mountainous catchment by calculating Specific Stream Power (SSP) and comparing spatial variability in Specific Stream Power as controlled by longitudinal variations in slope versus interannual climatic variability associated with ENSO. Finally, we study the possible climatic and geomorphic triggers of two partial avulsions that occurred in the basin during 2010 and 2011 to demonstrate how the sedimentary record preserves a complex mixture of local and far field environmental signals of the Cauca River.
Section snippets
Regional setting
The Cauca River drains some ~60,000 km2 and traverses >900 km in the northern Andes Mountains, flowing north along the Romeral Fault Zone, between the Western and Central Cordilleras (Fig. 1). As the river exits the intermontane valley, it enters the Mojana Basin (also known as the Lower Magdalena Valley Basin), a sedimentary basin undergoing active tectonic subsidence (Smith, 1986). The Cauca River joins the Magdalena River in the Mojana Basin and eventually flows into the Caribbean Sea. These
Methods
For this study, we divided the Cauca River system into two main segments: (1) the portion of the river that drains the mountainous catchment, and (2) the lowland portion of the river that flows through the Mojana Basin (Fig. 1C). The mountainous catchment is located above 200 m of elevation between the Central and Western Cordilleras of the northern Andes and is further subdivided into three regions from south to north, i.e., the upper, middle and lower parts of the catchment. This subdivision
Rainfall, runoff and wetland water-level variability
Monthly precipitation exhibits a bimodal regime in the upper, middle and lower parts of the Cauca River catchment (Fig. 3). Furthermore, the wavelet analysis of precipitation revealed that the upper, middle and lower Cauca River catchment exhibits semiannual, annual, 2–7-yr and 8–16-yr periodicities (Fig. 4). In the upper Cauca River catchment, the semiannual and annual periodicities are strong, whereas in the middle Cauca River catchment the semiannual periodicity is stronger than the annual
Climate variability in the mountainous catchment of the Cauca River (source) and the Mojana sedimentary basin (sink)
Our findings provide insights into the temporal frequencies of surface processes that govern mountain erosion and lowland deposition in tropical rivers. Environmental factors with different periodicities control runoff in the Cauca River catchment, as well as water levels in wetlands of the Mojana sedimentary basin. Precipitation and wetland water levels in the Mojana Basin show a strong annual periodicity (Fig. 3, Fig. 4, Fig. 8), related to their location near the northernmost extent of the
Conclusions
Analyses of the hydrology and geomorphology of the mountainous Cauca River and the Mojana sedimentary basin show that they are part of the same source-to-sink, coupled sedimentary system, but that the two regions are governed by different patterns of intra- and inter-annual climate variability. Biannual passage of the Intertropical Convergence Zone over the watershed, annual incursion of the Choco Jet, and sub-decadal influences of ENSO variability all serve as controls on precipitation, runoff
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank G. Weismann, J.P. Galve and S. Paternity (University of New Mexico) for discussions. B. Wilkinson, M. Wiesner, N. Zaremba and M. Brenner provided feedback on an early version of the manuscript. NP was financed by the Smithsonian Tropical Research Institute (internship), the 2018 National Geographic Early Career Grant (EC-51182R-18), the 2018 AAPG Grants in Aid, the 2019 GSA Graduate Student Grant, the K.D. Nelson Fund from the Department of Earth Sciences, the Syracuse University
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