|The first step in the watershed prioritization
process involved the delineation of watershed boundaries at an appropriate
scale. Watersheds boundaries needed to be hydrologically defined, consistent
with USGS eight-digit hydrologic unit codes, and based on a framework linked
to version 3 of the USEPA river Reach File (RF) coverage. Using these general
criteria, the USGS eight-digit hydrologic catalog units (51 in Illinois)
were further sub-divided into smaller scale watersheds in a manner compatible
with first version of the USEPA Reach File in 1987. At the same time, the
Illinois EPA developed a statewide database for tracking designated use
assessment information for Section 305(b) of the CWA (the Illinois
Water Quality Report) called the State Water Body Tracking System (WBTS).
A link was established between the Illinois EPAs stream coding system
and RF1. This link allowed WBTS designated use assessment data to be reach
indexed to a RF1 level of detail, and increased Illinois EPAs ability
to target watersheds based on decisions that were data driven.
RF2 (the second version of the Reach File) maintained
the integrity of RF1 and added one level of tributaries from the USGS
Geographic Names Information System (GNIS). These additions were limited
to streams of at least three miles in length and more than one-half mile
from an existing RF1 intersection. RF2 doubled the number of stream miles
found in RF1.
RF3 (the third version of the Reach File), maintained
stream reach designations from earlier reach file versions, and incorporated
USGS 1:100,000 scale Digital Line Graph (DLG) data into the Reach Files
network configuration. This created an endpoint at every intersection,
and included such geographic features as roads, map edges, and stream
confluences. The level of detail added in RF3 made it a viable tool for
Geographic Information Systems (GIS) applications.
In 1995, Illinois EPA utilized GIS technology to
reach index the WBTS assessment data for 305(b) to a RF3 level of detail
while simultaneously redelineating the 832 watershed boundaries (Figure
1) to a 1:24,000 scale. This allowed for an electronic link between the
WBTS data and RF3, which greatly enhanced Illinois EPAs ability
to target watersheds based on data driven decisions.
Figure 1. Illinois EPA Delineated Watersheds