Spatial and temporal variability in baseflow and stream drying in the Mattole River headwaters

Graduation Date

2015

Document Type

Thesis

Program

Other

Program

Thesis (M.S.)--Humboldt State University, Natural Resources: Forestry, Watershed & Wildland Sciences, 2015

Committee Chair Name

Andrew Stubblefield

Committee Chair Affiliation

HSU Faculty or Staff

Keywords

Streams, Watershed geomorphology, Baseflow, Stream fragmentation, Watershed, Streamflow recession, Humboldt State University -- Theses -- Natural Resources, Low flow, Climate change

Abstract

Pronounced seasonality in rainfall and consequently streamflow are natural components of a Mediterranean climate, and increases in human population, water use, and climate change have the potential to further increase water stress and scarcity in Mediterranean ecosystems in coming decades. Despite the importance of dry-season streamflow to aquatic ecosystems, our current understanding of the landscape features that exert dominant controls on baseflow generation remains limited. This study used repeated synoptic measurements of streamflow and surveys of the extent of dry reaches in small streams (DA 5.5 km2) in coastal Northern California to characterize variability in baseflow and patterns of reach drying, and to explore correlations of streamflow with basin characteristics. A nearby continuously recording streamgage was used as an index gage to calculate exceedence flows and to allow for the comparison of tributary flows measured on multiple dates. At 72-96% exceedence flows at the downstream gage, measured tributary yields ranged from 0.23 to 0.00 mm day-1. Tributary unit-area yields varied widely, and this variation increased as flows declined to near-zero at most sites. In nested basins, downstream declines in both discharge and unit-area yield were common, especially at the driest conditions encountered. In some salmonid-bearing streams, riffles lacked surface flow for at least 25% of the year. Most reaches showed no clear tendency towards "drying down" or "drying up". Instead drying appeared to emanate from dry "nodes" in the reach, and proceeded both upstream and downstream from that node as flows decreased. A specific discharge of 0.1 mm day-1is sufficient at either the upstream or downstream end of the surveyed reaches to maintain connected flow throughout the reach, although the relationship between flow and dry reach extent was highly variable. While the strength of correlations varied, in general basins with greater summer flow and a slower baseflow recession had steeper slopes, higher elevations, less flat ground and narrower valleys, more dissected and strongly convergent topography, and more precipitation. The magnitude of difference in water yield among basins was much greater than the difference in precipitation, suggesting that flow differences were the result of a combination of differences in basin water inputs, storage capacity, and routing. The positive correlation between basin steepness and flow is likely attributable to the key role of the thickness of the weathered bedrock layer in water storage, and more rapid bedrock weathering in steeper basins with higher rates of uplift resulting in greater basin storage capacity. Differences in internal plumbing within these basins potentially sets limits on their potential for baseflow increases resulting from management actions. However, the relationship between streamflow and the extent of dry channel in the study streams also demonstrates that even small changes in flow can lead to large changes in longitudinal connectivity of the channel network and habitat availability. Study results show that basins within a small geographic area (85 km2) and with fairly similar geology, vegetation, and topography may generate widely differing amounts of baseflow, independent of diversions and consumptive water use. Conversely, streams with naturally low baseflows are particularly susceptible to water diversion. The summer- rearing potential of basins with naturally higher baseflow per unit area will have greater resiliency to climate change effects, and preserving and enhancing habitat in these basins should be a priority. These results also suggests that in similar geologies low-gradient streams essential for coho salmon rearing may be particularly susceptible to climate change or water diversions that reduce streamflow.

https://scholarworks.calstate.edu/concern/theses/j6731599z

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