## 3 Results
### Effects of Scale on Burial/ISA Relationships  
Relationships between percent stream burial and ISA for the 4 scales of analysis (Table 3) are shown in Figure 7. In each case, the data exhibit a roughly 1:1 relationship below a level of approximately 30% ISA, above which most, if not all analysis units exhibit a greater amount of stream burial per unit ISA. This general relationship between burial and ISA is consistent for all analysis units, regardless of scale, across the full range in ISA cover, though not surprisingly, analyses with smaller-scaled units yield a larger number of units above the 30% ISA barrier. At the county-level, units above 30% ISA (Figure 7A) include Washington, DC, Arlington County, and the independent cities of Alexandria and Manassas, Virginia. These regions of the study area (Figure 8) have the longest history of urban development, spanning back to mid-late 18 th century, and have experienced continuous development and redevelopment to the present time. Of these units, only Manassas has maintained a roughly 1:1 ratio of burial to ISA, while the others exhibit significantly higher levels of burial per unit ISA. Only two subwatersheds (Figure 7B) exhibit greater than 30% ISA, their area corresponding with the most densely-developed regions of Washington, DC, and its first tier suburbs (Figure 8). In general, burial-ISA relationships for subwatersheds vary little from 1:1. At an analysis scale of 45 km\({}^{2}\) units, those grid cells with the highest ratio of burial to ISA (Figure 7C, orange ovals) still center on Arlington County, Alexandria, and the heart of Washington, DC (Figure 8). In addition, exhibiting >30 ISA, and elevated burial/ISA ratios at this analysis scale are \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Analysis Units** & **N** & **Mean Area (Range)** & **Area with \textgreater{}30\% ISA** \\ \hline Counties and Independent Cities & 16 & 632.1 km\({}^{2}\) (5.19–1728.50) & 292.01 km\({}^{2}\) \\ Subwatersheds & 166 & 89.9 km\({}^{2}\) (33.75–170.44) & 221.48 km\({}^{2}\) \\
45 km\({}^{2}\) grid cells & 283 & 45 km\({}^{2}\) & 450.24 km\({}^{2}\) \\
22.5 km\({}^{2}\) grid cells & 534 & 22.5 km\({}^{2}\) & 517.41 km\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistics for four scales of analysis unit across the study area. Figure 6: Percent burial as a function of impervious cover for the sixteen study counties. Colors of the points correspond with colors of development tiers in Figure 2. The dashed line represents the 1:1 line for burial and impervious cover. significant areas of the first-tier DC suburbs in Maryland and Virginia. Interestingly, there are two grid units with just over 30% ISA (Figure 7C, green oval) that exhibit significantly lower ratios of burial to ISA, centering on the cities of Herndon and Sterling in the western Suburbs of Virginia, and in suburban Prince George's County, Maryland, to the southeast of Washington, DC These areas are characterized by dense residential and commercial development centered on the local transportation corridors of VA Route 28 and the Dulles Airport Access Road (Rt. 267), and on the Interstate 95/495 corridors, respectively (Figure 8). Finally, at the smallest analysis scale of 22.5 km\({}^{2}\), the greatest number of analysis units exhibit increased burial/ISA ratios above the threshold of 30% ISA (Figure 7D). Those with the greatest deviation from 1:1 again include grid units spanning areas of Arlington, Alexandria, and the core of Washington, DC (orange ovals). There is also a large cluster of units above the 1:1 line, consisting of additional portions of DC and close suburbs with a longer history of development and infill. Those cells falling just above, and on the 1:1 line include large portions of the Potomac River, and other park areas with significant undeveloped area.