1448  2002 Estuarine Research Federation Estuaries Vol. 25, No. 6B, p. 1448–1456 December 2002 Downstream Effects of Restored Freshwater Inflow to Rincon Bayou, Nueces Delta, Texas, USA TERRY A. PALMER,PAUL A. MONTAGNA*, and RICHARD D. KALKE University of Texas Marine Science Institute, 750 Channel View Drive, Port Aransas, Texas 78373 ABSTRACT: Construction of two dams in 1958 and 1982 decreased freshwater inflow to the Nueces River and Nueces Delta marsh, which has become a reverse estuary where salinity increases upstream rather than downstream as would occur in a normal estuary. In 1995, an overflow channel was dug to breach the banks of the Nueces River to restore inundation of the marsh via Rincon Bayou, which is the main stem channel of the delta. Previous studies demonstrated a restoration of a normal salinity gradient and positive affects on benthos in the upper reaches of Rincon Bayou. The present study was performed to determine how far downstream the overflow channel had beneficial effects. A transect of eight stations was established and sampled quarterly between October 1998 and October 1999. Benthic characteristics were measured to assess ecological change. There were 12 precipitation and freshwater inflow events between the month preceding and ending sampling. The largest were in fall and restored normal salinity patterns. The macrobenthic com- munity was different in three zones. The upper four stations (6.4 km from the overflow channel) were highly variable in water inundation and salinity. The broad salinity range caused the lowest diversity and allowed short-lived pioneer species Streblospio benedicti, Laeonereis culveri, and Chirominid larvae to dominate. Biomass blooms occurred during fall inflow events in the upper reaches. The central stations (11.4 to 14.3 km from the overflow channel) were brackish, had more narrow salinity ranges, and were more diverse. Mulinia lateralis, Mediomastus ambiseta, Cerapus tubularis, and Am- pelisca abdita were dominant species. The third zone, the lower portion of Nueces Bay (27 km from the overflow channel), was distinct from other stations in the transect, because it had the greatest marine influence. It is in a later successional stage, with a more diverse community of larger organisms, e.g., Polydora caulleryi, Tharyx setigera, and Mysella planulata, which were dominant species and contributed to the larger biomass there. Overall, the overflow channel restored normal salinity patterns in the upper reaches (14 km) of Rincon Bayou and freshwater pulses ( 10 6 m 3 ) in fall increased benthic productivity indicating ecological functions were restored as well. This restoration however, only occurs inter- mittently for short time periods because the freshwater events are isolated and inflow volumes during the current study were too small to affect the lower Rincon Bayou or Nueces Bay. Introduction A growing demand for freshwater in the Nueces River watershed has caused a dramatic decrease in freshwater inflow into the Nueces Delta over the last 50 years. Average annual inflow to the delta has been reduced by 99% since 1958 when the Nu- eces River was first dammed (Irlbeck and Ward 2000). Before damming, the Nueces Delta was in- undated by freshwater inflow events that caused high water levels in the Nueces River to spill into Rincon Bayou, which is the main stem channel for the Delta. The upper part of the Delta (i.e., upper and central Rincon Bayou) were dead-end lakes without a direct connection to the Nueces River. Delta inundation occurred by over-banking of Nu- eces River flood waters and tidal inundation dur- ing isolated storms and tropical cyclones (Bureau of Reclamation 2000). The increasing demand for freshwater has reduced volumes and elevation of flood events so that natural marsh inundation was reduced to an average of one event every three * Corresponding author; tele: 361/749-6779; fax: 361/749- 6777; e-mail: paul@utmsi.utexas.edu. years (Irlbeck and Ward 2000). Decreasing fresh- water inflow has caused the Nueces Delta marsh to become a reverse estuary. This occurs when salinity increases upstream rather than downstream as would occur in a normal estuary. When freshwater inundation decreased, low salinity water from the Nueces River entered the marsh only upon rising tides and diluted water in the hypersaline marsh areas. The hypersaline marsh was created by evap- oration of unflushed pools. Hypersaline conditions have had adverse effects on the ecology of the Nu- eces Delta and on commercially and recreationally important shrimp and oyster populations (Monta- gna et al. 1998; Riera et al. 2000). The Rincon Bayou Demonstration Project was undertaken to determine if it was possible to ame- liorate environmental effects of decreasing fresh- water inflow (Bureau of Reclamation 2000). The project approach was to lower the minimum flood- ing threshold of the upper Nueces Delta from 1.64 m above mean sea level to about 0 m to increase the opportunity for larger and more frequent quantities of freshwater and tidal water to be di- verted into upper Rincon Bayou. The minimum