Date of Award

Summer 7-16-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Hoke, Gregory D

Second Advisor

Lautz, Laura K.

Subject Categories

Earth Sciences | Geomorphology | Hydrology | Physical Sciences and Mathematics

Abstract

Streamflow in high mountain and seasonally cold regions follows annual patterns that can make water quality and availability in these areas uniquely susceptible to changes in climate and land use. Precipitation in the form of snow accumulates during the winter months and gradually melts in the spring, leading to elevated streamflow that serves important ecological and human needs. This simple winter storage and spring release dynamic is especially sensitive to warming temperatures, which can result in a greater fraction of winter precipitation falling as rain and an earlier arrival of the hydrograph center of timing, with cascading effects on surrounding ecosystems. Changes in land use can similarly disrupt the natural storage and release balance by altering the pathways in which water is routed through a system. These changes necessarily impact the ways in which sediment and other solutes are delivered through stream systems. This dissertation investigates material fluxes at the watershed scale in high mountain and seasonally cold regions in an effort to better understand the processes impacting their magnitude, timing, and release and the broader implications for the management of water resources. Chapter 1 investigates these dynamics in the present day through the lens of a small urban headwater stream in the Northeastern United States characterized by frequent winter snowfall. In the surrounding watershed, sodium chloride is used seasonally as a deicing agent on roads and sidewalks, as is common in colder regions. Excess chloride loading poses a problem, however, because it can compromise sources of drinking water and threaten the health of surrounding ecosystems. The timing of chloride storage and release can also be altered in urban environments through the installation of armored stream channels and by increases in impervious surface cover. To investigate these landscape changes on chloride transport and storage, two years of streamflow and chloride concentration data were used to create continuous chloride load estimates for two contrasting reaches of an urban stream. The upstream reach is characterized by channelization and armored banks and is largely disconnected from groundwater while the downstream reach flows through a riparian floodplain and has a strong groundwater connection. Results from this study show that chloride loads in the channelized reach were similar to chloride application rates in the surrounding watershed. In contrast, chloride loads in the downstream reach were 50% lower than those delivered from upstream due to stream-groundwater interactions and water losses through subsurface flow paths. These findings show that longitudinal load estimates can be helpful in identifying areas of chloride storage and release, the magnitude of which may not always be apparent in urban settings. Chapters 2 and 3 consider both future and past influences on catchment scale sediment flux in the high Andes Mountains of Argentina and Chile. In this region, high mountain glaciers buffer streamflow during drier times of the year and water sourced from the uplands is critical to serving the needs of millions of people living in downstream communities such as Mendoza, San Juan, and Santiago. However, the high Andes are tectonically active, and sediment loads in regional streams and rivers can be high, posing a threat to the surrounding environment and human infrastructure alike. The area is also expected to face increases in temperature and decreases in precipitation in the coming decades. As a response, reservoirs have been built throughout the Andes, although the length of their usable lifespans is impacted by rates of sediment accumulation. In Chapter 2, future changes in streamflow and catchment scale sediment flux in the high Andes are modeled based a suite of end-member climate projections for temperature and precipitation in the coming decades. Results from this study show that reductions in precipitation propagate into even larger decreases in streamflow and sediment flux, although results from scenarios modeling warming without a change in precipitation were much more variable. In Chile, where annual precipitation is concentrated in the winter months, warming lead to an increase in high magnitude streamflow events as more storms that would have delivered snow in the high Andes delivered rain instead. These events were also associated with high sediment loads and could be connected to an increased risk in geohazards such as landslides. In Argentina, however, where the Andes act as an orographic barrier to winter storm events originating from the Pacific, the streamflow and sediment flux response to warming was more akin to simulations with reduced precipitation. These results show that different water management strategies may be needed on either side of the Andes in the coming decades. Chapter 3 takes a different approach, and investigates how sediment moves through the landscape over much longer timescales by utilizing cosmogenic radionuclides in several watersheds in the Argentine high Andes. First, 10Be in river sand was used to estimate catchment-wide erosion rates at the millennial scale and compared to decadal scale erosion rates from steam gauge data, with comparisons generally showing favorable agreement. Erosion rate estimates were also used to predict how long water reservoirs sited at the foot of the Andes can be expected to last before becoming filled with sediment, with predictions from 10Be for five different reservoirs occupying a fairly narrow range from 129 to 127 years. Finally, 10Be samples were paired with an analysis of in situ 14C in an effort to investigate sediment transport dynamics from the hillslope to catchment outlet. Low 14C/10Be ratios in this study suggest that despite agreement between decadal and millennial scale erosion rates, sediment transport times through these complex watersheds may range from a minimum of 10,000 to 19,000 years. This finding further indicates that sediments carried by streams out of the mountain front in the present may have been eroded from high elevation hillslopes in the early Holocene or even the late Pleistocene, connecting events in the geologic past to processes impacting humans today.

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