Effects of 2007 Drought on the Cahaba River
As of mid-October, 2007, there have been 59 days that streamflow in the Cahaba River at Centreville (about the midpoint of the river) has been lower than any recorded flow since 1901 (there were records kept for 79 years out of that interval), according to the USGS website, http://waterdata.usgs.gov/nwis/rt . Clearly, this has been an epic drought.
However, stream ecologists point out that rivers need to experience the full range of hydrologic conditions in order to stay healthy, so a drought has some positive aspects along with the negatives:
Here are a few beneficial effects of low flow:
- Fewer stormwater toxins from non-point sources wash into the river.
- Stream banks have an opportunity to grow vegetation without being scoured, thus stabilizing the banks.
- Turbidity levels are lowest during base flow times.
- Less sediment is washed into the river.
- Sediment flushes out of the thalweg, the deepest, fat-flowing portions of a stream channel.
There are also a lot of negative impacts from prolonged drought. Some of these have been made more extreme because of human-induced impacts:
- Low flow provides less aeration, resulting in very low dissolved oxygen levels (This effect on aquatic wildlife is made worse by high temperatures, which often accompanies a drought).
- Freshwater mussels and fishes are at greater risk of predation during low, clear flow intervals.
- Freshwater snails are more frequently stranded and desiccated during low flow intervals.
- Filamentous algal growth is not scoured from the stream bed, allowing it to accumulate and smother habitat. Blue-green algae species (which often release toxic waste products) often grow better than Green, Golden-brown, and Yellow-green algae species under drought conditions.
- Little or no flushing of sediment from the streambed (outside of the thalweg).
- Fish populations can be more genetically isolated by low stream levels.
- There may literally be less extensive habitat available for aquatic wildlife.
- Pollutants from point source discharges, like wastewater treatment plants, are more concentrated than at any other time.
Algae growth can smother essential aquatic habitat during extended low flow periods.
