ELDORADO DO SUL, Brazil, May 7 (Reuters) - Rescuers

rushed to evacuate people stranded by devastating floods across

the southern Brazilian state of Rio Grande do Sul on Tuesday,

with at least 90 dead, thousands left homeless, and desperate

survivors seeking food and basic supplies.

On the outskirts of Eldorado do Sul, 17 kilometers (10.5

miles) from the state capital of Porto Alegre, many people were

sleeping on the roadside and told Reuters they were going

hungry. Entire families were leaving on foot, carrying

belongings in backpacks and shopping carts.

"We've been without food for three days and we've only just

got this blanket. I'm with people I don't even know, I don't

know where my family is," said a young man who gave his name as

Ricardo Junior.

The flooding has hampered rescue efforts, with dozens of

people still waiting to be evacuated by boat or helicopter from

stricken homes. Small boats crisscrossed the flooded town

searching for survivors.

The state's Civil Defense agency said the death toll had

risen to 90 with another four deaths being investigated, while

131 people were still unaccounted for and 155,000 homeless.

The heavy rains that began last week have caused rivers

to flood, inundating whole towns and destroying roads and

bridges.

Rain is forecast to let up on Thursday but then continue

through the weekend.

Climate experts attributed the extreme rainfall in Rio

Grande do Sul to the confluence of a heatwave caused by this

year's El Niño phenomenon, which warms the waters of the Pacific

and brings rain to southern Brazil; a weaker cold front with

rain and gales coming from the Antarctic; and unusual warmth in

the Atlantic also raising humidity.

Global warming exacerbates these phenomena and

intensifies the effects between such systems, making weather

unpredictable, said Marcelo Schneider, a National Meteorology

Institute (Inmet) researcher.

ELECTRICITY CUTS

In Porto Alegre, a city of 1.3 million inhabitants, downtown

streets were under water after the Guaiba River breached its

banks with record water levels.

Porto Alegre residents faced empty supermarket shelves and

closed gas stations, with shops rationing sales of mineral

water. The city distributed water in trucks to hospitals and

shelters.

The floods have also impacted water and electricity

services, with more than 1.4 million affected overall, according

to Civil Defense.

Almost half a million people were without power in Porto

Alegre and outlying towns as electricity companies cut off

supplies for security reasons in flooded neighborhoods. National

grid operator ONS said five hydroelectric dams and transmission

lines were shut down due to the heavy rains.

The city's airport, its apron under water, has suspended all

flights since Friday.

Fuel shortages were reported as state-run oil company

Petrobras said it was having trouble moving diesel from its

refinery in badly-flooded Canoas within metropolitan Porto

Alegre, a senior government official said.

President Luiz Inacio Lula da Silva said on a government

television program that the extent of the damage will not be

known until the waters subside. He promised federal aid for the

state in what is considered its worst ever climate disaster.

JP Morgan economists projected that the impact of the floods

on Brazil's economy would be a modest dent in GDP growth and a

marginal increase in inflation, mainly due to higher prices for

rice that is largely produced in Rio Grande do Sul. The

government said Brazil will import rice to stabilize the market.

Besides destroying critical infrastructure, the heavy rains

and flooding have left grains fields under water and killed

livestock, interrupting the soy harvest and halting work at

multiple meat plants.

The Rio Grande port, a major port for grain exports, was

operating normally, the state's port authority said. However,

main access roads were impassable, disrupting grain deliveries

to the port as trucks had to make a wide detour, exporters said.