IFFEZHEIM, Germany, July 26 (Reuters) - Germany's biggest river power plant at Iffezheim on the Rhine needs sustained rainfall in late summer to avoid a repeat of some recent years when output was curtailed by 20%-25%, its manager told Reuters television on Wednesday.

"We have had a dry phase of around eight weeks in June, July and hope that rain returns because then we will see the river Rhine and other rivers in Baden-Wuerttemberg filling up again," said Ulrich Rost of EnBW, which runs the plant jointly with France's EDF.

Rain this week has already raised water levels again but the river is still shallow in some parts as a result of dry weather in the past two months, disrupting barge traffic and threatening supplies to some industries.

With an installed capacity of 140 megawatts (MW), the Iffezheim plant produces 840 gigawatt hours of carbon-free electricity a year if there is enough water so that it can integrate output of its turbines with services to the shipping industry.

It must help ships get enough room via a complex weir system that also benefits agriculture.

When precipitation and the level of snowmelt coming through the Rhine from the Alps up to its mouth in the northern Netherlands ease, the plant can lose power output to the degree seen in 2008 and 2022, Rost said.

Hydropower accounts for 10% of the energy mix of EnBW and production shortfalls in that segment have to come from other fuels such as coal, gas and renewables like solar and wind to meet delivery obligations.

The Rhine also serves as a main artery for the inland shipping of goods and its water is used to cool thermal power generation plants dotted along parts of the waterway and its estuaries.

(Reporting by Frank Simon und Andreas Buerger ; Writing by Vera Eckert, editing by David Evans)