Improved River Slope Datasets for the United States Hydrofabrics

The CONtiguous United States (CONUS) “Flood Inundation Mapping Hydrofabric - ICESat-2 River Surface Slope” (FIM HF IRIS) dataset integrates river slopes from the global IRIS dataset for 117,357 spatially corresponding main-stream reaches within NOAA’s Office of Water Prediction operational FIM forecasting system, which utilizes the Height Above Nearest Drainage approach (OWP HAND-FIM) to help warn communities of floods. To achieve this, a spatial joining approach was developed to align FIM HF reaches with IRIS reaches, accounting for differences in reach flowline sources. When applied to OWP HAND-FIM, FIM HF IRIS improved flood map accuracy by an average of 31% (CSI) across eight flood events compared to the original FIM HF slopes. Using a common attribute, IRIS data were also transferred from FIM HF IRIS to the CONUS-scale Next Generation Water Resources Modeling Framework Hydrofabric (NextGen HF), creating the NextGen HF IRIS dataset. By referencing another common attribute, SWOT vector data (e.g., water surface elevation, slope, discharge) can be leveraged by OWP HAND-FIM and NextGen through the two resulting datasets. The spatial joining approach, which enables the integration of FIM HF with other hydrologic datasets via flowlines, is provided alongside the two resulting datasets.

If you use this dataset in your work, please cite: 

Chen, Y., Cohen, S., Baruah, A., Devi, D., Dhital, S., Tian, D., & Munasinghe, D. (2025). Merging Remote Sensing Derived River Slope Datasets with High-Resolution Hydrofabrics for the United States. Scientific Data, 12(1), 1657. https://www.nature.com/articles/s41597-025-05941-6

1. Version History

FIM HF IRIS Version 1 - Version 3: Based on OWP HAND-FIM HF v4.5.2.11, SWORD v16 and IRIS v2.6.

NextGen HF IRIS Version 1 - Version 3: Based on FIM HF IRIS v1.0 and NextGen HF v2.2, joined through the common fields, i.e., id, in both hydrofabrics.

FIM HF IRIS Version v4: Based on OWP HAND-FIM HF v4.8.6.1, SWORD v17 and IRIS v3.2, joined using the improved RiverJoin spatial framework for better join performance (more complete extraction and fewer wrongly extracted reaches).

FIM HF IRIS Version v5: Based on OWP HAND-FIM HF v4.8.6.1, SWORD v17c and IRIS v3.3, joined using the RiverJoin spatial framework. The joined flowlines have been further refined by removing unexpected low-order tributaries, i.e., first- and second-order reaches that are not properly extracted. This version also adds the SWOT-observed slope (from the latest SWORD v17c) as an additional slope source.

2. Variable Description

2.1 FIM HF IRIS v5

HydroID: Unique identifier for FIM HF stream reaches

feature_id: The NWM feature identifier of the corresponding FIM HF flowline

From_Node: Upstream node ID

To_Node: Downstream node ID

reach_id: The SWORD reach identifier

avg_combined_slope_iris33: The IRIS v3.3 average combined (across- and along-track) slope joined to the corresponding FIM HF flowline [mm/mm]

slope_swd17c: The SWORD v17c slope joined to the corresponding FIM HF flowline [mm/mm]

sobs_adj_p50_swd17c: The SWORD v17c adjusted median SWOT-observed slope (sobs_adj) joined to the corresponding FIM HF flowline [mm/mm]. This is the 50th-percentile slope from the SWOT slope observations with negative values set to zero.

3. Files in v5

Both files contain the same 115,888 refined FIM HF reaches and the same eight attributes described in Section 2.2. They differ only in whether the geometry is included.

FIMHf_IRIS_SWORD_v5.gdb — Esri file geodatabase, layer FIMHf_reaches_IRIS_SWORD_v5. Requires ArcGIS Pro 3.2 or newer (or GDAL 3.6+), as reach_id is stored as a 64-bit integer.

FIMHf_IRIS_SWORD_v5.csv — Attribute table only, without geometry. Provided for use outside GIS software, e.g. in Python, R or spreadsheet tools, and can be linked back to the geodatabase or to any other FIM HF layer through HydroID and feature_id.

4. Reference

Scherer D., Schwatke C., Dettmering D., Seitz F.: ICESat-2 river surface slope (IRIS): A global reach-scale water surface slope dataset. Scientific Data, 10(1), 359, 10.1038/s41597-023-02215-x, 2023.

Gearon, J. H., Altenau, E. H., Pavelsky, T. M., Durand, M. T., Collot d'Escury, N., Yang, X., Malaterre, P.-O., Frasson, R. P. d M.& Bendezu, L. (2026). SWOT River Database (SWORD) (Version v17c) [Dataset]. Zenodo. https://doi.org/10.5281/zenodo.21415370

Scherer D., Schwatke C., Dettmering D., Seitz F. : ICESat-2 Based River Surface Slope and Its Impact on Water Level Time Series From Satellite Altimetry. Water Resources Research, 10.1029/2022WR032842, 2022.