The Mahantango Formation can be found in the watershed of North Branch Mahantango Creek and is named for the creek. Most of the North Branch Mahantango Creek watershed is mountainous, with spurs and ridges. Its channel is sinuous and runs through rock formations consisting of limestone, sandstone, and shale. North Branch Mahantango Creek is a limestone stream. The annual precipitation in the watershed of North Branch Mahantango Creek is , although it ranged from in the early 1920s. The wettest month is June, when of precipitation falls. The driest month is February, when falls. The total annual runoff is and the annual streamflow is . Streamflow occurs more in March than in any other month (). The months with the least streamflow are July, August, and September, with of streamflow each. The creek's temperature is upstream of Mount Pleasant Mills. The watershed of North Branch Mahantango Creek has an area of approximately 37 square miles. According to the Susquehanna River Basin Commission, the creek's watershed is one of the highest-quality watersheds in the lower Susquehanna River watershed. The most common land use in the impaired part of the watershed of North Branch Mahantango Creek is deciduous forest, which makes up 55.5% of the area. Cropland makes up 21.1% of this part of the watershed and hay and pastures make up 17.3% of it. 3.7% is mixed forest, and 2.5% is coniferous forest. 0.5% of the land (a quarry) is considered "high-intensity development" by the Pennsylvania Department of Environmental Protection. 0.2% of the land is considered "low-intensity development" by the Pennsylvania Department of Environmental Protection. An unimpaired portion of the creek's watershed includes of 58.5% deciduous forest, 21.1% cropland, 11.9% hay and pasture, 4.2% coniferous forest, and 3.2% mixed forest. The remaining 0.7% of this part of the watershed's area consists of 0.5% "low-intensity development" and 0.2% "high-intensity development". Nearly all of the northern part of the impaired portion of the watershed of North Branch Mahantango Creek is forested and most of the southern part is agricultural or developed. A nearby unimpaired part of the creek's watershed has the same layout. Both parts of the watershed have wetlands.