Hyatt, T., Seixas, G. and Ramsden, K., 2022. Methods description for the Skagit River lidar-derived hydrography. Skagit River System Cooperative, La Conner, WA. pp. 11.

Analysis of fish populations and fish habitats in the Skagit River watershed has been hampered
for decades by the lack of mapped hydrography that accurately depicts the existence and location
of streams in a digital geographic information system (GIS) format. Without accurate
hydrography it is impossible to meaningfully evaluate the shading effects of riparian vegetation
on streams at a spatial scale relevant for watershed-level restoration planning, and difficult to
map the location, extent, and distribution of anadromous and resident fish. Similarly, locating
fish-blocking culverts at road crossings is often ambiguous when mapped streams do not
correspond with the actual topography, and mapping protective stream buffers is inaccurate at
best, and likely much less effective, when maps either include streams that don’t exist on the
ground, or don’t include streams that do exist. Lidar-derived hydrography (LDH) does not
completely eliminate this last problem, but it does improve the locational accuracy of most
streams.
In 2016 the Swinomish Tribe and Skagit County contributed to a Washington DNR effort to
collect lidar data that, when combined with previous lidar, would fully encompass Skagit
anadromous zones. The Skagit River System Cooperative (SRSC), an environmental protection
and restoration consortium of the Swinomish Indian Tribal Community and the Sauk-Suiattle
Indian Tribe, has since constructed an updated hydrography layer that combines new lidarderived
hydrography where it can be generated, with existing hydrography from the National
Hydrography Dataset (NHD) in areas where the NHD is as good or better than can be generated
from lidar. Thus in the headwaters where lidar does not yet exist for the Skagit, the NHD was
retained. Likewise, the NHD was retained in the mainstems and many lowland areas where the
NHD has been recently updated using aerial photography, particularly in the agricultural
drainages. In the many miles of small and intermediate streams, particularly those in forested
areas, the new lidar hydrography provides distinctly superior stream arcs over the NHD and
other available hydrography layers.
This document is a description of the data sources and methods used to create the new Skagit
LDH and incorporate within it the existing NHD hydrography.