However, the sample results have shown some trends between river flow levels and high bacteria counts. The data shows that just after rainstorms and when flow levels are rising the bacterial counts rise and may exceed the recommended levels for contact recreation such as swimming. This is likely caused by runoff from creeks and other tributaries carrying animal waste and other pollutants into the Rio Grande. This occurs primarily during the summer monsoon season, between June and October, but can happen at any time of year. Bacteria, such as Fecal Coliform and E. Coli can cause illness if ingested.
Children are particularly susceptible to bacterial infection.
Small children are more likely to be exposed through splashing, and otherwise getting river water in their eyes, mouths, ears or nose. On the other hand, during periods of prolonged low flows, the bacterial levels tend to be very low, and well within safe limits of state standards for recreation. During low flows, the river tends to be high in salts as is common in desert rivers below dams. The high salinity may reduce the amount of bacteria in the water during low flows. Because many of the small communities along the river do not have adequate sewage treatment facilities, there may be bacteria in the water immediately downstream of these towns even during periods of low flow.