The website utilizes hydrology data from the United States Geologic Survey and Mexico’s National Institute of Statistics and Geography to provide continuous coverage for the study area.

Sources:

Many features of the website utilize the Hydrologic Unit Code (HUC) System (Watershed Boundary Dataset, WBD) developed by the United States Geological Survey (USGS) with some minor variations explained below. We also display the USGS National Hydrography Dataset (NHD), which depicts lentic and lotic waters of the state. To learn about these datasets visit the USGS website.

Unfortunately, the USGS has not created HUCs throughout the study area, into Mexico. HUCs cover a small part of Mexico extending from U.S. HUC’s, across the border a short distance into Mexico. This is no surprise since the system was developed for the United States, but in order to expand our study area into Mexico (major addition for Version 3 of the website and database) we had to implement a similar system that could work in concert with the USGS HUC system. Mexico’s National Institute of Statistics and Geography (INEGI) created a similar, albeit completely independent, system with three levels of nested hydrologic connected areas. We used their lowest ‘subcuenca’ level, which we though sufficiently similar to USGS HUC 8s in size to provide a complete hydrological watershed boundary dataset for the study area.

Edits to United States HUC and Mexican subcuenca systems:

We utilized most HUC levels, from small HUC 12s to larger HUC 6s as shown in our map tab’s layers. However, lower level (larger shapes), which combine Texas' major river basins, seemed of little use for users of our data. Instead, we combined HUC 8 and Mexican subcuencas into major river basins that we thought most biologists interested in aquatic ecosystems would find useful. See our Major River Basin layer in the map tab – this layer is also our official study area.
Allowing overlapping hydrology systems, which we initially preferred, proved to be problematic for a number of reasons, so we decided to edit the shapes of U.S. HUCs along the Rio Grande that overlapped with Mexico. Thus the Rio Grande is unique because those U.S. HUCs along the Rio Grande were clipped to meet the Mexican subcuencas, which stop immediately at the Rio Grande. Users should understand that checklists and HUC assignments to occurrences are based on these edited shapes. Furthermore, the Rio Grande, cannot be treated as having hydrologically nested HUCs standard across the US portion of the study area.
Small areas on the borders of some Mexican subcuencas were determined to be errors and were removed.
The names of Mexican subcuencas were edited by removing the "R" prefix they all had in common and replacing it with "MEX".

How are hydrologies used:

The current version of the website displays the NHD dataset as a layer on the map tab, but the NHD is not utilized in other ways. Mexican subcuencas and the USGS’ WBD HUC system levels 12 through 6 are used throughout the website, together forming a merged continuous hydrology covering the entire FoTX study area that is used for summarizing and displaying data as described below:

HUC8s/subcuencas can be viewed on the map tab as independent layers alongside species occurrences.
Checklists derived from occurrences are available for all HUC8s/subcuencas.
Native layers for all native species are described as compilations of HUC8s/subcuencas.
Tableau dashboards summarize the data by HUC8s/subcuencas.
Native Fish Conservation Areas are compilations of HUC8s/subcuencas.
All records in the database have been assigned HUC8s/subcuencas based on their coordinates. This is how records are called up in hydrology-based queries from the data tab. Thus, records lacking coordinates, also lack HUC8/subcuencas assignments and are not discoverable using hydrology-based queries. We made HUC8/subcuencas assignments regardless of the size of the occurrence’s spatial error estimate. It is thus conceivable for records with large errors, and especially so for occurrences that are located close to HUC boundaries, to be assigned to HUC8s/subcuencas in which the true occurrence did not occur. Assignment of other categorical assignments (county or NFCA) work the same way.
Our major basins are compilations of HUC8s/subcuencas into biogeographically meaningful drainages. They are used on our query page and visible on our map tab as a layer. They are similar to maps produced by the Texas Water Development Board and other state agencies, but may differ primarily on how coastal sub-basins are grouped.

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