Harmful algal blooms (HABs) are overgrowths of algae or cyanobacteria in water and can be harmful to humans and animals directly via toxin exposure or indirectly via changes in water quality and related impacts to ecosystems services, drinking water characteristics, and recreation. While HABs occur frequently throughout the United States, the driving conditions behind them are not well understood, especially in flowing waters. In order to facilitate future national model development and characterization of HABs, this data release publishes a synthesized and cleaned collection of HABs-related water quality and quantity data for river and stream sites across the United States. It includes nutrients, major ions, sediment, physical properties, streamflow, chlorophyll and other types of water data. This data release contains files of harmonized data from the USGS National Water Information System (NWIS). Continuous sensor data for 132 parameters (35 of which returned data) between January 1, 2005 and December 31, 2022 were downloaded from NWIS programmatically. All data were harmonized into a shared format and grouped by generic parameters; data are in files named "daily_{parameter_grp}.csv", and the "pcode_group_xwalk.csv" maps parameter codes to parameter groups. Lastly, we include a "site_metadata.csv" containing site identification and location information for all sites with water quality and quantity data, and mappings to the National Hydrography Dataset flowlines where available. This work was completed as part of the USGS Proxies Project, an effort supported by the Water Mission Area (WMA) Water Quality Processes program to develop estimation methods for PFAS, harmful algal blooms, and metals, at multiple spatial and temporal scales.