Cold river water can be tempting on a sweltering summer day, but before wading into the Souhegan, it would be wise to check the river’s E. coli levels.

Based on testing June 26, bacteria levels were up.

Except for the headwaters in New Ipswich and at Watson Park in Merrimack, which was retested last week, the river’s entire length had been considered unfit for swimming.

The high bacteria levels were probably the natural result of recent conditions, said George May, of the Souhegan Watershed Association, because very dry weather was followed by big rainstorms that washed pollutants into the river.

Recent retesting at Watson Park in Merrimack; however, might mean E. coli levels have dropped since June 26 when 30 SWA volunteers spread out the entire length of the Souhegan, as well as the Merrimack River between Manchester and Tyngsborough, Massachusetts, and collected water samples to be analyzed for bacteria, dissolved oxygen, temperature and turbidity.

In Amherst, the levels of E. coli from the June 26 testing were more than 20 times what they should be for safe swimming.

Levels less than 88 per 100 milliliers are considered clean and satisfactory for swimming.

Levels reaching 126 are OK for healthy adults; 126 to 406 is poor and not recommended for swimming by the SWA but would still meet state standards. Higher than 406 is not acceptable for swimming at all.

On June 26, near the Horseshoe, a popular Wilton swimming area, the number was a bit over 137 and at the Pine Valley Mill in Milford it was 275.

Behind Hayward field in Milford, it had jumped to 816 and at the Swing Bridge it was at 1,986.

At the Amherst Country Club it was an extremely high 2,419.

At Watson Park, the June 26 number had been more than 800, but the SWA retested the area last week and results came back July 3 at 99 – “almost clean,” said May, who founded the SWA more than 22 years ago.

The lower number is good news, because people swim there on the July 4 holiday, he said. Because of Watson Park’s popularity, the water is tested every week and warnings are posted on a kiosk and there is a warning flag.

E. coli bacteria is found in the digestive tracts of people, birds and animals, and its presence in rivers can come from agricultural runoff, wildlife, pets and septic systems. High levels can cause swimmers’ ear and gastrointestinal problems, especially in children or vulnerable adults.

In the Merrimack River, E. coli levels were much lower because it is a much bigger river with a substantial flow that can dilute pollution, according to the SWA.

But none of the Merrimack River sites tested between Manchester and Tyngsborough June 26 met the level of cleanliness for completely safe swimming.

Until the river reached Nashua, the Merrimack’s levels were in the 100 range, but once in Nashua, the levels increased to dangerous levels, according to the SWA.

Taylors Falls Bridge was over 900; the site at Sagamore Bridge was over 1,700. By the time the river reached Tyngsborough the pollution had been diluted enough to return to the 100 level.

Despite the rain, the river flows are still below where they should be this time of year, the SWA reported.

Dissolved oxygen levels are a good measure of the overall health of the river, and both rivers almost always register at a very healthy level for oxygen.

Volunteers will be out again in mid-July, and information is available at www.souheganriver.org.

Kathy Cleveland can be reached at 673-3100 or kcleveland@cabinet.com.