A campaign, protest movement that could yet block one of East Africa's most significant infrastructure projects, has netted Ms Angelei one of this year's Goldman Prizes, one of the highest annual honours for grassroots environmental activists.
Not that there are too many grass roots in the Turkana region of Kenya, on the border with Ethiopia, South Sudan and Uganda. Rains have been infrequent as far back as communal memory stretches and have become even scarcer in recent years.
The region's prized water resource is Lake Turkana.
It's one of those unusual "endorheic" lakes that has no outflow; water that flows in, and is not used directly, either evaporates or percolates down into aquifers, which in turn provide water for those pastoralists who keep their herds some way distant from the lake itself.
Now, the Ethiopian government is building a major dam, GIBE-3, on the Lower Omo River just over the border. The Omo currently provides about 80% of Lake Turkana's water.
Nearly 240m high, the dam would generate 1,870MW at full flow. It would become the biggest dam in Africa, and the fourth-largest in the world.
Prime Minister Meles Zenawi says it must be built "at any cost" to help Ethiopia electrify and develop, and to power the irrigation schemes and plantations he wants to establish along the Omo valley.
The Kenyan government, which is likely to buy some of the electricity, also supports the scheme.
But Ikal Angelei fears the dam could dry the lifeblood for hundreds of thousands of people in the river valley and around Lake Turkana - lowering the water level by many metres, increasing the already high salinity, and preventing the drainage into aquifers that keeps cattle alive kilometres away.
"Communities that have been benefiting from the flow and the pasture will all have to move to where water is available - you're creating pressure for conflict in an area that already has a high potential for conflict because of scarce resources," she tells BBC Environment correspondent, Richard Black.
"You do anything to change the current balance, you really exacerbate conflict in the region."
The recently increased aridity, combined with population growth, has already brought conflict, some of it across borders.
In 2006, when parts of Turkana had already gone three years without rain and three million Kenyans were on daily food aid, the correspondent visited the region briefly.
A local governor talked of pastoralists crossing the border with Uganda in search of somewhere to water their animals, exchanging fire with Ugandan air force planes; hardly a recipe for stability between neighbours.
Last year (2011) saw conflict between the Kenyan Turkana people and the Ethiopian Merille.
Ms Angelei does not downplay the development benefits that GIBE-3's electricity could bring, though she does dispute Ethiopian government claims that its impact on Lake Turkana will be negligible.
What she's asking for first is to have the issues discussed thoroughly and openly, with all factors on the table.
The Ethiopian government awarded contracts for building GIBE-3 without tendering - and when this was pointed out to potential funders including the World Bank, European Investment Bank and African Development Bank, they withdrew.
Kenyan law has been another battleground.
She explains that the government is obliged to consider the needs of local people when considering such projects; and as the dam's first Environmental Impact Assessment didn't even mention Lake Turkana and was kept unpublished for years, she argues the government has broken its own law. A case is currently in the courts.
Turkana is among the "cradles of humanity", where our species lived and evolved; so the area has World Heritage interest as well.
Perhaps the most important aspect of the campaign she's mounted with her group Friends of Lake Turkana (FoLT) consists of talking to the communities that make up a variegated and neglected social landscape with a literacy rate barely above 10%.
In 2008, she found, hardly anyone she talked to had heard of GIBE-3 even though it was already underway; many did not even know what a dam was. So much for community engagement and informed consent.
A year later, chiefs and tribal elders signed a declaration authorising FoLT to tackle the Kenyan government on their behalf.
She argues that the governments are looking at the potential economic benefits of the project without understanding its economic costs.
No-one has yet studied the net worth of the fish Lake Turkana produces, the benefits of the cattle pasture - and the costs that society will have to bear if those things disappear.
And there are questions too of whether hydroelectric schemes are the best way to power a region that regularly sees droughts and may in future see them even more frequently as a consequence of man-made climate change.
In 2003, 2008 and 2009 Ethiopia saw regular power cuts because of droughts affecting its existing hydro dams, including GIBE-1 and GIBE-2, which also sit on the Omo River.
"Our prime minister said two years ago that we cannot afford to depend on hydro - but now we're going to buy hydroelectricity from Ethiopia, which is even drier than Kenya," she says.
There's a certain irony in noting that just as Ethiopia is encouraging plantations along the Omo basin of cotton, one of the thirstiest crops and not exactly necessary for food, farmers in Australia's drought-hit Murray-Darling basin are being pressured to give it up.
Success is not assured for Ikal Angelei's campaign.
But even if it fails, it will have created a more informed local society, brought governments closer to the position of having to observe their own environmental laws, and made it more difficult to finance projects with dubious credentials.