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data. The SPDSS Task 106 memorandum evaluated the consequences of eliminating the HydroBase <br />stream table that was created only for the Water Information Sheet feature in the CWRAT software, while <br />retaining the more traditional wd_water table, which is used to link water bodies to structures and water <br />rights. The stream table includes features that cross water district boundaries, whereas the wd water <br />table contains records for features in each water district (and therefore some streams are represented in <br />multiple records). The Task 106 memorandum recommended removing the HydroBase stream table. It <br />may now be more appropriate to focus on linking point features in HydroBase with NHD until the issue <br />of whether a statewide stream layer matching HydroBase water sources can be addressed. <br />Limitations of the existing stream and wd water tables in HydroBase include: <br />• The data in the stream and wd water tables do not include spatial information (i.e., the linework <br />to draw the stream) and no existing spatial data layer exists that matches the data in the tables. <br />Consequently, currently there is no graphical representation of the data. <br />• The "trib to" information for the tables is often missing or inaccurate. The stream mile data, <br />which can be used to indicate the point at which a tributary joins another stream, is often null or <br />zero. Therefore, a stream network cannot be constructed for queries or to understand the spatial <br />proximity of streams. <br />• The records are often for a specific water district and therefore multiple records are included for a <br />feature, reflecting the fact that the data are used primarily for administration and traditionally <br />have not had a physical representation (i.e., graphical). <br />• The spatial resolution and names of features may not be sufficiently precise for some uses. For <br />example, a stream may have been identified because a water right is associated with it; however, <br />an upstream tributary may not have a water right and is therefore not included in HydroBase. <br />• The geoloe.str_mile field is designed to indicate at which point on a stream a structure is located. <br />However, these values are often missing or contain zero values. Therefore, although structures <br />are often correctly located with a stream, it is difficult or impossible to determine the relative <br />upstream order of structures using HydroBase. <br />In summary, because of the limitations of stream data in HydroBase, data can only be used for simple <br />displays. <br />2.2 NHD Availability <br />The NHD is distributed from the USGS website http://nhd.usgs.~ov, which provides the following <br />overview: <br />"The National Hydrography Dataset (NHD) is a comprehensive set of digital spatial data <br />that contains information about surface water features such as lakes, ponds, streams, rivers, <br />springs and wells. Within the NHD, surface water features are combined to form "reaches, " <br />which provide the framework for linking water-related data to the NHD surface water <br />drainage network. These linkages enable the analysis and display of these water-related data <br />in upstream and downstream order. <br />The NHD is based upon the content of ZISGS Digital Line Graph (DLG) hydrography data <br />integrated with reach-related information from the EPA Reach File Version 3 (RF3). The <br />NHD supersedes DLG and RF3 by incorporating them, not by replacing them. Users of DLG <br />or RF3 will find the National Hydrography Dataset both familiar and greatly expanded and <br />refined <br />Page 2 of 17 ~IZlverslde Technnlo~yv Inc. <br />4s':rkv Resuurce.s Ergmee: mg end ~C9 nsuNrng <br />