Network Model

The network model examines spatial relationships among roads, river networks, and public land in interior and south-central Alaska. Our analysis is supported by several GIS data layers. The river network is modeled using the National Hydrography Dataset (USGS 1999); the Alaska Department of Transportation GPS centerline dataset provides a detailed road network (Alaska DOT 2006); and the boundaries of federal and state lands were obtained from an Administrative Boundaries dataset (Alaska DNR 2001).

National Hydrography Dataset (NHD)

The NHD is managed by the USGS and EPA. It is a comprehensive set of digital spatial data that contains information about surface water features such as lakes, ponds, streams, rivers, springs, and wells. Within the NHD, surface water features are combined to form “reaches,” which provide the framework for linking water-related data to the NHD surface water drainage network. These linkages make it possible to analyze and display these water-related data in upstream and downstream order (USGS 1999). We started with the assumption that all migration of invasive plants on the river system would be downstream. We then utilized the NHD as our base layer for river networks, since it covers the entire state at a fine scale (1:63360), and also incorporates flow network information allowing us to efficiently identify all river reaches downstream from individual road crossings.

Road Network

There is no comprehensive road network dataset available for Alaska, but the Alaska Department of Transportation (ADOT) provided a draft version of a new GPS road centerline network. It covers the contiguous highway system plus Kodiak and Cordova. Some roads are not included in this dataset—for example, the Denali National Park road, State of Alaska-administered logging roads, and private roads—but this road data is the best currently available.

Public Conservation Units

We began with the land classification boundaries identified on the Administrative Large Parcel Boundaries dataset from December 20, 2001, produced by the Alaska Department of Natural Resources (Alaska DNR 2001) (figure at right). We selected parcel types to include in our model by reviewing the agency affiliations of land managers who have been active in Alaska’s Committee for Noxious and Invasive Plant Management (CNIPM) over the last few years. Managers who are already aware of and concerned about invasive plants on the public lands they manage may be most likely to benefit from the information provided by this work. We refer to that list of parcel types in aggregate as “conservation units.” Land management objectives and agency attitudes and approaches toward invasive species vary widely among these conservation units.

Road-river interfaces

The interface between roads and rivers (or streams) can take several forms. Roads can cross, run parallel to, and dead-end at rivers. We defined “crossings” as intersections between the NHD-derived streams-and-rivers network and the ADOT road centerline network. At this phase our analysis does not include the parallel or dead-end road-river interfaces, except for rare occasions where small spatial mismatches cause the river and road linework to intersect. NHD sub-region 1904 (the Alaska portion of the Yukon River drainage) yielded a total of 919 crossings. Where roads cross major rivers or streams there is typically a bridge; we refer to these points on the landscape as “major crossings.” Small streams or ephemeral drainages typically pass beneath roads in culverts; we refer to these points as “minor crossings.” To associate river reaches downstream from crossings with conservation units, the Administrative Large Parcel Boundary dataset was intersected with river reaches downstream from crossings (after the river reaches were buffered by 100 m).