This document, upon approval by the U.S. Environmental Protection Agency (EPA), establishes a Total Maximum Daily Load (TMDL) for sediment in the Seneca Creek watershed (basin number 02140208) (2010 Integrated Report of Surface Water Quality in Maryland Assessment Unit ID: MD-02140208). Section 303(d) of the federal Clean Water Act (CWA) and the EPAs implementing regulations direct each state to identify and list waters, known as water quality limited segments (WQLSs), in which current required controls of a specified substance are inadequate to achieve water quality standards. For each WQLS, the State is required to either establish a TMDL of the specified substance that the waterbody can receive without violating water quality standards, or demonstrate that water quality standards are being met (CFR 2009b). The Maryland Department of the Environment (MDE) has identified the waters of the Seneca Creek watershed on the States 2010 Integrated Report as impaired by sediments (1996, Clopper Lake - 1998), nutrients phosphorus (1996, Clopper Lake - 1998, and Little Seneca Lake - 1998), chlorides (2010), ammonia (2010), and impacts to biological communities (2002) (MDE 2010b). The designated use of the Seneca Creek mainstem and its tributaries is Use I-P (Water Contact Recreation, Protection of Aquatic Life, and Public Water Supply), except for 1) Little Seneca Creek and its tributaries, from the outlet of Little Seneca Lake to the streams confluence with Bucklodge Branch, and Wildcat Branch and its tributaries, which are designated as Use III-P (Nontidal Coldwater and Public Water Supply), and 2) Little Seneca Creek and its tributaries upstream of Little Seneca Lake, which are designated as Use IV-P (Recreational Trout Waters and Public Water Supply) (COMAR 2009a,b,c,d,e,f). The TMDL established herein by MDE will address the Maryland 8-digit (MD 8-digit) watershed 1996 sediments listing, for which a data solicitation was conducted, and all readily available data from the past five years have been considered. A Water Quality Analysis (WQA) for eutrophication to address the MD 8-digit watershed nutrients/phosphorus listing was approved by the EPA in 2009. A TMDL of phosphorus and sediments for Clopper Lake was approved by the EPA in 2002, and a WQA for eutrophication to address the Little Seneca Lake nutrients/phosphorus listing was approved by the EPA in 2006. The general listing for impacts to biological communities was removed due to a stressor identification analysis completed in 2009, and as a result, , the 2010 Integrated Report now identifies chlorides, ammonia, and sediments as specific stressors impairing aquatic life (MDE 2010b). The Seneca Creek watershed aquatic life assessment scores, consisting of the Benthic Index of Biotic Integrity (BIBI) and Fish Index of Biotic Integrity (FIBI), indicate that the biological metrics for the watershed exhibit a significant negative deviation from reference conditions based on Marylands biocriteria listing methodology. The biocriteria listing methodology assesses the condition of MD 8-digit watersheds by measuring the percentage of sites, translated into watershed stream miles, that are assessed as having BIBI and/or FIBI scores significantly lower than 3.0 (on a scale of 1 to 5), and then calculating whether this percentage differs significantly from reference conditions.