The dataset documents the spatial and temporal variability of nutrients and related water quality parameters at high spatial resolution in the North Delta, Central Delta, and the Western Delta out to Suisun Bay in the Sacramento-San Joaquin River Delta of California, USA. The dataset includes nitrate, ammonium, phosphate, dissolved organic carbon, temperature, conductivity, dissolved oxygen, and chlorophyll as well as information about phytoplankton community composition. Data-collection cruises were conducted under three different environmental/flow conditions in May, July, and October of 2018. The data release consists of a xml document, 13 text/csv documents, and a zip file. Descriptions for each document and file are listed below: 1. METADATA: (2018_High_resolution_mapping_surveys_v2.0.xml) – Metadata for child item “Assessing spatial variability of nutrients and related water quality constituents in the California Sacramento-San Joaquin Delta at the landscape scale: 2018 High resolution mapping surveys.” 2. QA DATA: (Delta Water Quality Mapping 2018 Data Corrections and Offsets.csv) – Data corrections and offsets calculations applied to the datasets. 3. DATA DICTIONARY (Delta Water Quality Mapping 2018 Data Dictionary v2.0.csv) – Data dictionary for tables and attributes. 4. DISCRETE SAMPLING DATA: (Delta Water Quality Mapping 2018 Discrete Sampling v2.0.csv) – Discrete sampling, water chemistry data from the Sacramento-San Joaquin Delta study area. 5. PHYTOPLANKTON ENUMERATION DATA FROM DISCRETE SAMPLES: (Delta Water Quality Mapping 2018 Discrete Sampling Phytoenumeration v2.0.csv) – Discrete sampling, phytoenumeration data from the Sacramento-San Joaquin Delta study area. 6. May 2018 FIELD MAPPING DATA (High resolution, timestamped and 20 second median filtered): (Delta Water Quality Mapping May 2018 High Resolution.csv) – High resolution, in situ, unfiltered, in vivo fluorescence (IVF), water chemistry data from the Sacramento-San Joaquin Delta study area. All measurements were collected in the field. 7. May 2018 MAPPING DATA ON COMMON SPATIAL FRAMEWORK: (Delta Water Quality Mapping May 2018 Spatially Aligned.csv) – Spatially Aligned (Centerline Extracted) and interpolated, water chemistry data from the Sacramento-San Joaquin Delta study area. 8. July 2018 FIELD MAPPING DATA (High resolution, timestamped and 20 second median filtered): (Delta Water Quality Mapping July 2018 High Resolution.csv) – High resolution, in situ, unfiltered, in vivo fluorescence (IVF), water chemistry data from the Sacramento-San Joaquin Delta study area. All measurements were collected in the field. 9. July 2018 MAPPING DATA ON COMMON SPATIAL FRAMEWORK: (Delta Water Quality Mapping July 2018 Spatially Aligned.csv) – Spatially Aligned (Centerline Extracted) and interpolated, water chemistry data from the Sacramento-San Joaquin Delta study area. 10. FIELD MAPPING DATA (High resolution, timestamped and 20 second median filtered): (Delta Water Quality Mapping October 2018 High Resolution.csv) – High resolution, in situ, unfiltered, in vivo fluorescence (IVF), water chemistry data from the Sacramento-San Joaquin Delta study area. All measurements were collected in the field. 11. October 2018 MAPPING DATA ON COMMON SPATIAL FRAMEWORK: (Delta Water Quality Mapping October 2018 Spatially Aligned.csv) – Spatially Aligned (Centerline Extracted) and interpolated, water chemistry data from the Sacramento-San Joaquin Delta study area. 12. CALCULATED DIFFERENCES BETWEEN MAY AND JULY, 2018: (Delta Water Quality Mapping May to July 2018 Differences.csv) – Calculated differences between months for water chemistry data from the Sacramento-San Joaquin Delta study area. 13. CALCULATED DIFFERENCES BETWEEN MAY AND OCTOBER 2018: (Delta Water Quality Mapping May to October 2018 Differences.csv) – Calculated differences between months for water chemistry data from the Sacramento-San Joaquin Delta study area. 14. CALCULATED DIFFERENCES BETWEEN JULY AND OCTOBER, 2018: (Delta Water Quality Mapping July to October 2018 Differences.csv) – Calculated differences between months for water chemistry data from the Sacramento-San Joaquin Delta study area. 15. SHAPE FILES FOR ALL RESULTS: (Maps_Shapefiles.zip) – Zip file containing 117 shapefiles for spatially aligned water quality constituents from the Sacramento-San Joaquin Delta study area. 16. METHODS: (Methods_ChildItem_v2.0.pdf) – Text document describing methods used for data and sample collection and analysis.