Spatial patterns of watershed impervious cover relative to stream location

The urban stream syndrome may not be limited to streams in urbanized watersheds. We measured the spatial pattern of impervious cover in ∼82,800 small watersheds across the conterminous United States by comparing watershed-based and stream-based measures of imperviousness. The watershed-based measure was the commonly used watershed percentage impervious cover. The stream-based measure was the percentage of watershed stream length flowing through impervious cover. Spatial pattern of impervious cover was classified on a watershed basis as proximal to streams, distal to streams, and uniform by comparing the two measures of impervious cover. We used a classification threshold of ±5% to assign watersheds to the three classes (i.e., stream-based minus watershed-based ≥5% = proximal; watershed-based minus stream-based ≥5% = distal; else = uniform). We then applied the classification to two impervious cover thresholds, ≥5% and ≥15%. For ≥5% and ≥15% thresholds, impervious cover was distributed uniformly across ∼70% and ∼86% of the watersheds, respectively. For the remaining watersheds, the proximal spatial pattern was ∼12× and ∼4× greater than the distal spatial pattern for the ≥5% and ≥15% impervious cover thresholds, respectively. The proximal spatial pattern of impervious cover occurred predominantly in non-urbanized watersheds, resulting in a widespread occurrence of a relatively high percentage of streams flowing through relatively high impervious cover in watersheds where the total percentage impervious cover was relatively low. The spatial pattern of change in impervious cover between ca. 2001 and ca. 2006 did not avoid streams. Impervious cover increased in the vicinity streams in ∼55% of the watersheds with increases in impervious cover. During this period, the length of streams flowing through ≥5% and ≥15% impervious cover increased by ∼9800 km and ∼6900 km, respectively.