The goal of this study was to develop a suite of inter-related water quality monitoring approaches capable of modeling and estimating the spatial and temporal gradients of particulate and dissolved total mercury (THg) concentration and particulate and dissolved methyl mercury (MeHg) concentration in surface waters across the Sacramento / San Joaquin River Delta (SSJRD). This suite of monitoring approaches included: a) data collection at fixed continuous monitoring stations (CMS) outfitted with in-situ sensors, b) spatial mapping using boat-mounted, flow-through, sensors and c) satellite-based remote sensing. The focus of this specific Child Page is to present all data collected during the underway boat mapping component of the study. The data compiled here represent measurements from a suite of field sensors collected as part of a flow-through system (FTS) integrated into a high-speed boat monitoring package. Data was collected during four mapping cruises representing seasonal conditions between October 2019 and May 2021. Sensor measurements were made at a 1-second frequency while underway at speeds of up to 35 mile per hour (mph) along three nautical routes crossing the SSJRD. Each mapping event was conducted over a three-day period centered on a LANDSAT8 satellite overpass (Landsat 8 | U.S. Geological Survey (usgs.gov)). All measurements were geo-referenced using a GPS running concurrently with the boat FTS and were merged in real time during data collection using a datalogger. The measurements collectively provide information about the amount and character of the substances in the water that relate to mercury species concentration and distribution throughout the Delta. The package consisted of two flow-paths containing debubblers and flow controllers to ensure consistent and comparable measurements were obtained from multiple sensors deployed both in series and in parallel along the two flow-paths. The first flow-path (referred to as the FTS flow-path) pulled river water through a 179-micron screen to protect the instruments from large particles. The flow-path included an EXO2 (YSI EXO2 Multiparameter Water Quality Sonde | ysi.com), a thermosalinigraph (SBE 21 SeaCAT Thermosalinograph | Sea-Bird Scientific - Overview | Sea-Bird (seabird.com)), fluorescent dissolved organic matter (fDOM) and chlorophyll fluorometers (ECO Fluorometer | Sea-Bird Scientific - Overview | Sea-Bird (seabird.com)), a submersible ultraviolet spectrophotometer (SUNA V2 Nitrate Sensor | Sea-Bird Scientific - Overview | Sea-Bird (seabird.com)), a transmissometer (C-Star Transmissometer | Sea-Bird Scientific - Overview | Sea-Bird (seabird.com)), and an algal classification sensor (FluoroProbe (Chlorophyll) - bbe moldaenke (10cells.com)). The sensors collectively provide information about the optical properties of the dissolved and particulate substances in the water and correspond to similar measurements conducted at the CMS locations used in this study. The second flow-path (referred to as the ACS flow-path) pulled river water in the same manner as the FTS flow-path, but the water was further split into two paths, one that contained a 0.2 micron filter and the other that did not contain a filter. Each flow-path was equipped with a spectrometer (ac-s Spectral Absorption and Attenuation Sensor | Sea-Bird Scientific - Overview | Sea-Bird (seabird.com)) capable of measuring attenuation and absorption spectra throughout the visible wavelength range. This set-up allowed for the simultaneous measurement of absorption (a) and attenuation (c) within the filter-passing (dissolved) fraction and the particulate fraction of the water.