A Bowling Green, Ky., firefighter careens through the rapids of the Barren River on April 1, after trying to assist in the rescue of two teenage fishermen.

A Bowling Green, Ky., firefighter is rescued by a fellow firefighter after he careened through the rapids of the Barren River, on April 1 during a river rescue.

Zack Sowders and Gage Lee, both 16, of Bowling Green, Ky., wait to be rescued on April 1, after their boat overturned in the rapids of the Barren River while they were fishing.

Long-tailed mayflies (Palingenia longicauda) fly over the surface of the Tisza river near Tiszainoka, 84 miles southeast of Budapest on Wednesday. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year.

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszainoka.

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszainoka.

A paddler negotiates white water during the first day of the Dusi Canoe Marathon in Pietermaritzburg on Thursday. More than 1700 competitors take part in the annual 75-mile race which begins in Pietermaritzburg and ends in Durban.

Spectators watch as participants canoe by on day two of the Dusi Canoe Marathon in Pietermaritzburg on Friday.

Paddlers portage dangerous sections of the river during day two of the Dusi Canoe Marathon in Pietermaritzburg on Friday.

A paddlers cools off at a water point during day two of the Dusi Canoe Marathon on Friday.

A boat is stranded on dry land along the Danube river bank in Belgrade on Dec. 2, 2011. Record low water levels have pushed up power prices across the Balkans and may force local utilities to further boost expensive power imports to meet demand.

Vessels carrying more than 1,000 tons were blocked in this area of Belgrade earlier this week because the water level was lower than normal. For river traffic to resume water levels would have to rise by at least 19 inches, but meteorological services forecast no rain in the coming days.

Oil swirls in a flooded gravel pit in Lockwood, Mont., after a pipeline break early Saturday, July 2. The ExxonMobil pipeline that runs under the Yellowstone River near Billings in south-central Montana ruptured and dumped an unknown amount of oil into the waterway, prompting temporary evacuations along the river.

Students compete in the traditional punt race on the Neckar river in the southern German city of Tuebingen on June 23, 2011. The boats are up to 12 metres in length with each with eight crew members.

A street sign is seen in floodwaters from the rising Mississippi River in St. Francisville, La., where a dozen homes and businesses, and several camps, were flooded, on Friday, May 20, 2011. Residents were leaving in the face of a mandatory evacuation order set to kick in on Saturday as Mississippi River water flowing through the Morganza spillway is expected to reach communities in the Atchafalaya Basin.

An old school bus sits in floodwaters from the rising Mississippi River in St. Francisville, La., where a dozen homes and businesses, and several camps, were flooded, on Friday, May 20, 2011.