Date of Award

Winter 1-31-2021

Document Type

Thesis: Open Access

Degree Name

MS Marine Science

Department

Environmental and Ocean Sciences

Committee Chair

Dr. Suzanné C. Walther

Committee Member

Dr. Zhi Yong-Yin

Committee Member

Eric M. Cathcart

Abstract

Rivers in semi-arid climates are directly influenced by local geographic and hydrologic conditions and impacted by modifications to hydrology via urbanization. Changes can influence erosion, morphology, habitat sustainability, and watershed health. In highly urbanized southern California coastal regions, these rare open spaces provide vital ecosystem services. Los Peñasquitos Creek in San Diego County is one such watershed. Using stream surveying and laboratory methods we quantified channel characteristics, grain size distribution, total metal concentration [M], organic carbon (%OC), and phosphate to longitudinally characterize the creek for improved management. Results identified three distinct reaches in the watershed (upper, middle, lower). Downstream, depth and velocity are inversely related (R2: -0.86), while grain size decreases (D50:45mm-0.2mm), influenced by slope-driven widening and overbank deposition in the middle reach. Phosphate and [M] vary, likely influenced by anthropogenic runoff. Data suggests that %OC (instead of grain size) is more strongly correlated with [M] overall, especially zinc and lead, and is influenced by riparian zone vegetation density. This study emphasizes the importance of local and geomorphic influences on geochemical variability. Suggestions include 5-year or drought year Cu, Hg, Pb, Zn monitoring (exceeded SQuirT screening) at specific sites and continued nutrient analysis for eutrophication at the confluence. This study also uses two methods to estimate and model runoff in future scenarios using HEC-RAS. The first method estimates recurrence intervals (RI) based on local PeakFQ data. Commonly used in hydraulic engineering and flood modeling, the RI method estimates flows as an average number of times a peak flow will occur over a return period (years). For Los Peñasquitos watershed, the estimated flows for the RI years shown are as follows: RI-5=2,899, RI-10=4,095, RI-25=5,675, RI-50=6,870, RI-100=8,066, RI-500=10,841 cfs. To address the impact on flow from precipitation and soil saturation, the second method uses a 5-day Antecedent Precipitation Index (API) and daily precipitation in a multiple regression empirical model to estimate runoff. Future projections for rainfall, based on climate models using Representative Concentration Pathways (RCP) are applied to observed data and interpolated to RCP 2.6, 4.5, 6.0, and 8.5 scenarios and resulted in 7,908-8,246 cfs within the next 50-100 years. We recommend using future projections for rainfall and land-use to better estimate flow and address projected trends.

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