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"GROUNDWATER", tributary and non-tributary groundwater, and wells. Lakes (42 records) and <br />reservoirs (7 records) typically are identified by specific names, although "RESERVOIRS" is used in one <br />record. In summary, the water bodies other than streams that are identified in HydroBase amount to <br />relatively few records. The procedure used to link HydroBase with spatial data for these features is <br />similar to that described below for stream features, but may involve different types of data (e.g., polygon <br />features instead of linework) and non-automated data checks may be more appropriate because of the low <br />number of records. <br />The stream features, which comprise the majority of the records in the HydroBase stream and wd water <br />table, do not appear to be represented by any spatial features linked to HydroBase. Although maps exist <br />for Colorado water bodies (e.g., the "Water Division, District, Designated Groundwater Basins and <br />Management District Boundaries" map prepared by the Office of the State Engineer), the linework for <br />such maps is available only in hardcopy or does not appear to include attributes that can be linked to <br />HydroBase. <br />A general automated approach, using linear referencing and NHD, can be used to link HydroBase stream <br />data to NHD. The matched stream linework will include foreign keys to link both HydroBase stream and <br />NHD attribute data. Linear referencing in ArcMap is done with the route event geoprocessing tool. This <br />tool can reference either point or polygon data to lines. Therefore, stream linework can be referenced to <br />NHD linework in two ways: referencing the endpoints of the stream linework or buffering the stream <br />lines to create a polygon and then referencing the resulting polygons. There are problems associated with <br />each approach. The problem with using the endpoints in an automated process is that a point is matched <br />to the nearest NHD reach, which may result in an incorrect match (e.g., due to sharp turns in a stream). <br />The problem with buffers in an automated process is that they extend the original stream line by the <br />buffer distance, thus adding references to NHD reaches that should not be included. The two approaches <br />can be evaluated and the appropriate procedure determined. This may involve using one of the two <br />methods or using both in a specific order to compliment each other. Regardless of the specific method <br />used, the outcome of the linear referencing procedure is a table with the following columns: <br />Stream ID -identifier of the stream used to create the buffer. <br />NHD Reach Code - identifier of the NHD reach that intersects the stream buffer. <br />From Measure -distance of the upstream intersection point from the downstream end of the NHD <br />reach as percent of the total reach length. <br />To Measure -distance of the downstream intersection point from the downstream end of the NHD <br />reach as percent of the total reach length. <br />This table can be used as the data source for a map layer that shows the referenced NHD segments. This <br />layer can be exported to a feature class of streams (i.e., stream layer) that match the NHD reaches. If any <br />streams did not intersect with the NHD reaches, they can be selected spatially and merged with the <br />matched streams to form a single feature class. Stream and NHD attribute data can be joined to the <br />matched streams on the stream ID and reach code, respectively. <br />In linear referencing, the linework from NHD is used to represent the spatial features. However, rather <br />than copying these features, the features of interest are referenced using attribute data. For example, to <br />represent stream X from HydroBase, the end-points of the stream are identified as being on NHD reaches <br />(using reach codes), utilizing measures (percentage of distance from reach end-points). The main issue <br />with linking NHD and HydroBase is that the stream reaches in HydroBase are not well defined. The <br />records in the stream table were created mainly for the CWRAT application and have not been quality <br />controlled. The records in the wd_water table do not represent full stream features. To link features with <br />NHD will require consolidation of the HydroBase records into something similar to the stream table, and <br />Page 10 of 17 ~fifversFde TeChnnlG~yv #nc. <br />4s':rkv Resuurce.s Ergmee: mg ane ~C.u ^sc?lrng <br />