Profile of The River

River Rangers collect various data which include physical, chemical, and biological properties of the San Marcos River. Baseline data has been established to compare to any unusual data results. Water Quality of the San Marcos River, a good reference for baseline data, was published in The Texas Journal of Science-Volume 49, no. 4. in November of 1997 by Alan W. Groeger and students of the Department of Biology at Southwest Texas State University.

Dissolved oxygen is one of the major parameters that indicates the health and vitality of the flora and fauna in a body of water. The sources of O₂ in water are generated by photosynthesis in daylight and by the exchange of air in a turbulent water surface. Because the levels of dissolved oxygen naturally fluctuate over a 24 hour period , it is essential to continually monitor at a specific time to detect any adverse changes.

Conductivity measures the relative mineral content of water by its ability to conduct electricity. In the San Marcos River calcium carbonate (aka limestone) is the predominant mineral and is fairly stable over time.

The pH test determines how acidic or basic the water is. The metabolic processes of animals are directly affected by pH levels. Fluctuations in pH occur naturally during a 24 hr period due to plant photosynthesis and plant and animal respiration. The mineral content of the San Marcos River minimizes changes in pH by providing a continuous buffer.

Air temperature varies from season to season and sometimes from year to year. The effects of air temperature on water are mainly noticed in shallow areas and near the water surface.

Water temperature in the headwaters of the river is a fairly constant 22°C (72°F). Downstream water temperature varies slightly with the input of tributaries within the river's watershed and the seasonal climatic changes.

The Secchi Disk reading provides total depth and water clarity information. The depth at which light can penetrate into water directly affects the amount of photosynthesis in submerged plants.

Nitrates and phosphates are nutrients that occur in water naturally in trace amounts. High levels may indicate a problem from agricultural areas, chemical fertilizers, or untreated sewage. Excessive nutrients may cause algal blooms that deplete O₂ from the water at night during respiration.

Field Observations include: how the water is being used either by humans or other animals; land use; the abundance or lack of vegetation in or on the banks of the water; water surface; evidence of erosion; sources of input (ie. parking lot or storm drain culverts); weather conditions. The types of observations should remain consistant to record changes over time.