In the Rocky Mountains, there is uncertainty about the source areas and emission types that contribute to nitrate (NO₃) deposition, which can adversely affect sensitive aquatic habitats of high-elevation watersheds. Regional patterns in NO₃ deposition sources were evaluated using NO₃ isotopes in five National Parks, including 37 lakes and 7 precipitation sites. Results indicate that lake NO₃ ranged from detection limit to 38 μeq/L, δ¹⁸O (NO₃) ranged from −5.7 to +21.3‰, and δ¹⁵N (NO₃) ranged from −6.6 to +4.6‰. δ¹⁸O (NO₃) in precipitation ranged from +71 to +78‰. δ¹⁵N (NO₃) in precipitation and lakes overlap; however, δ¹⁵N (NO₃) in precipitation is more depleted than δ¹⁵N (NO₃) in lakes, ranging from −5.5 to −2.0‰. δ¹⁵N (NO₃) values are significantly related (p < 0.05) to wet deposition of inorganic N, sulfate, and acidity, suggesting that spatial variability of δ¹⁵N (NO₃) over the Rocky Mountains may be related to source areas of these solutes. Regional patterns show that NO₃ and δ¹⁵N (NO₃) are more enriched in lakes and precipitation from the southern Rockies and at higher elevations compared to the northern Rockies. The correspondence of high NO₃ and enriched δ¹⁵N (NO₃) in precipitation with high NO₃and enriched δ¹⁵N (NO₃) in lakes, suggests that deposition of inorganic N in wetfall may affect the amount of NO₃ in lakes through a combination of direct and indirect processes such as enhanced nitrification.