A new nation, old wounds
Dawn was breaking over the White Nile as a heavily laden boat came into view.
One by one its haggard passengers disembarked, wading through the shallows to reach a muddy bank. Women struggled to avoid dropping infants into the brackish water, children helped by carrying heavy bundles, one man in civilian clothes splashed ashore with an AK-47 rifle.
All had just completed a perilous overnight journey to escape the besieged town of Bor in South Sudan.
The White Nile bisects this country, so civil war has caused a nightly exodus across its waters.
Every sunset, barges packed with refugees leave Bor on the east bank. When dawn breaks 12 hours later, the survivors land at a tiny harbour in the town of Minkamen on the western shore.
But the river has a treacherous current and fighting is raging nearby, so many fugitives do not complete the journey.
A crowded barge sank outside the northern town of Malakal on Sunday - apparently accidentally - drowning 200 people.
Here in Minkamen, the boatmen must be skilled navigators because the river is a ribbon of islands and channels perhaps 16km wide.
The barges go as slowly and silently as possible, dodging and weaving among the bulrushes like biblical fugitives.
The scale of this nightly crossing is remarkable. Some 85000 people have arrived in Minkamen in the past four weeks, overwhelming a resident population of about 60000.
South Sudan's civil war is exactly a month old and the upheaval is already catastrophic.
After only 31 days of fighting, one person in every 20 is a refugee within or outside the country. Some 400000 are "internally displaced", and another 75000 are in refugee camps in neighbouring states. Put simply, this is the first great humanitarian emergency of 2014.
South Sudan is the world's newest nation - having won independence from its northern neighbour in July 2011 - and its people surely hoped their unimaginable sacrifice might bring some reward. Instead, their country has torn itself apart within three years of its birth - and this in the space of four weeks.
Why did the killing start? And where will it lead?
To start with the "why": the confrontation pits Salva Kiir, the president, against a rebel army led by Riek Machar, a former vice-president who was sacked last July. These bitter rivals agree on nothing except that their war is a struggle for power.
Machar accuses the president of trying to become a dictator; Kiir claims his unruly subordinate started everything by trying to carry out a coup on December 15.
Not many impartial observers believe the coup story. Many believe Kiir has been searching for a pretext to deal with Machar, especially since the latter declared his aim to run for president in next year's election.
These two enemies have very old scores to settle. Both fought for independence as senior commanders in the rebel Sudan People's Liberation Army.
In 1991, Machar sought to seize the leadership of the struggle by trying to topple the Sudan People's Liberation Army's then commander, John Garang. He eventually achieved a public "reconciliation" with Garang in 2002.
But Garang died in a helicopter crash a few weeks after the war against Khartoum ended in 2005. Machar might have buried the hatchet with him, but never with Kiir, who took over as the rebel army's leader and became South Sudan's first president.
There is no point of principle at stake here: old political rivalries are causing the people of South Sudan to be killed and driven from their homes.
Of all the wars in recent history, this one must rank among the most futile.
But that makes it no less dangerous. Behind everything lies tribal rivalry between the largely Nuer rebels and the Dinka-led government. The bloodshed might have started as a political clash between two powerful men, but it is now escalating into an ethnic war.
Those who make the journey across the White Nile from Bor are mainly Dinkas, fleeing the predominantly Nuer insurgents .
One refugee, Achol Malual, whose uncle was killed in his home, said: "It is a tribal war. If they see you are Dinka, you are being killed."