## 2 Materials and Methods
### Mapping Potential Stream Burial  
Our predictions of stream burial for the entire PRB used recently generated \"potential\" stream maps (10-m resolution, [35], henceforth referred to as \"streams\" or \"stream maps\"), which represent a much more complete coverage of streams (including ephemeral and intermittent streams) than is represented by the more commonly used National Hydrography flow lines. The term \"potential streams\" arises from the fact that these stream maps were generated from topographic modeling and trained from observations of streams in forested settings. Therefore, they show our best understanding of where streams would be if the entire basin had the same land use history as modern forested lands. Therefore, the stream maps provided by [PERSON] et al. [35] allow identification of the potential stream network in areas that have been urbanized for many decades, including those streams that were buried prior to any available stream maps. Details on the methodology used to generate potential stream maps are available, including a detailed analysis of classification omission and commission error [35]. A remote-sensing derived impervious surface area (ISA) product was acquired from the 2006 National Land Cover Dataset [36] and was used to represent hard, urban surfaces. The overall approach we used was to identify buried streams as stream segments that intersect ISA above a defined threshold. The purpose of predictive modeling (next section) is to identify the threshold that most accurately separates the training data into buried and intact stream segments. In practice, the stream and ISA layers are created at different resolutions and geo-location accuracy. It is entirely possible for a stream to appear to flow through a high-ISA pixel, but to in fact be protected by a riparian buffer that is represented by an area of low ISA pixels a short distance away. To address these and similar spatial co-registration issues between stream and impervious datasets, we calculated a selection of  
Figure 1: The probability of stream burial was modeled for streams across the entire Potomac River Basin PRB), which spans portions of four states (MD, PA, VA, WV) and all of Washington, DC. statistics intended to represent the spatial variability (e.g., mean, standard deviation, range, minimum, maximum) of ISA surrounding each 10 m stream segment. The results of these statistical calculations were associated with each stream segment for use in subsequent burial modeling. Additionally, the flow accumulation area (FAC, discussed below) for each segment was calculated for use with predictive modeling, which serves as a proxy for stream size. To build a training dataset for mapping stream burial, representative, high spatial resolution (30-cm) aerial photographs (circa 2001, 2006), were obtained for 17 sample areas (mean area \(\sim\)50 km\({}^{2}\), sum \(>\)2% basin area) across the PRB, chosen to capture a diversity of environments, including the full gradient in land use from forest to urban. From the stream segments located within the bounds of the aerial photos, a stratified-random set of reaches (\(n=1620\)) was selected, with 10% of the points located within each 10% increment of ISA. Additional streams in the highest ISA category (90%-100%) were selected to ensure the training data included roughly equal numbers of buried and intact streams. The aerial photos were examined to determine the status of each selected stream reach as either buried or intact. If a stream was visible in the air photograph the stream was labeled as intact. If the stream flowed through a forested area, but the trees obscured the stream, the stream was also labeled as intact. However, if the stream was not visible due to urban land cover (pavement, buildings, or graded bare earth) the stream was labeled as buried. This technique is not sensitive to other types of stream burial in urban environments (e.g., buried streams piped and covered by vegetation under lawns or agricultural lands, or inundated by man-made impoundments).