Rain that lands on a Montana ranch, on a Pittsburgh street or on an Alberta wheat field can leave North America through the same river mouth below New Orleans. Mapping that funnel helps explain a dead zone in the Gulf.
41%
of the contiguous United States drains into the Mississippi River. That's about 1.2 million square miles spread over all or parts of 31 states and 2 Canadian provinces, emptying through a single delta.
THE FUNNEL: the Continental Divide in the west and the Appalachians in the east set the rim. Almost everything in between flows to one mouth.
01 · The catchment
One continent, one exit
From Montana to Pennsylvania, this one drainage system covers about two-fifths of the lower 48.
1.2M
square miles drained (≈3.2 million km²)
31 + 2
U.S. states and Canadian provinces inside the rim
≈2,350
miles from Lake Itasca, Minnesota, to the Gulf
41% drains to the Mississippi59% drains to the Pacific, Atlantic, Great Lakes, Hudson Bay or other Gulf rivers
02 · World ranking
The fourth-largest basin on Earth
Only the Amazon, the Congo and the Nile collect water from a larger area, and the Nile is barely ahead.
Drainage-basin area, million km². Mississippi–Missouri highlighted.
Hydrologists draw basin edges differently, so published areas vary by a few percent. Every major compilation still puts the Mississippi in fourth place, about the size of the Nile basin and larger than the Ob, which drains much of western Siberia.
03 · What the water carries
The funnel concentrates fertilizer
Water from the farms in the middle of the basin carries most of the nitrogen that reaches the Gulf.
Share of nitrogen delivered to the Gulf by source · USGS SPARROW, Alexander et al. 2008. *Computed remainder.
Agricultural sources supply more than 70% of the nitrogen and phosphorus delivered to the Gulf. Corn and soybean fields alone account for 52% of the nitrogen. Later USGS modelling places the highest nitrogen yields in the Corn Belt, centred on Iowa and Indiana, which is the dashed zone on the map, hundreds of miles from the sea.
04 · The dead zone
Every summer, the shelf runs out of oxygen
The nutrients feed algae blooms. When the algae die and decay, they use up the oxygen in the bottom water off Louisiana and Texas.
In 2025 the hypoxic zone measured 4,402 square miles, which put the five-year average at 4,755. That average is more than twice the Hypoxia Task Force's 2035 goal of under 1,900. The largest zone on record, 8,776 sq mi in 2017, was the biggest since mapping began in 1985. Weather drives much of the year-to-year swing: in 2020, Hurricane Hanna mixed oxygen back into the water just before the survey.
05 · The other end of the funnel
A delta that's dissolving
The river built coastal Louisiana from sediment. Levees now carry much of that sediment past the marshes and out to sea.
of coastal Louisiana land lost from 1932 to 2016 (±171), close to the size of Delaware
The rate has slowed. A football field's worth of land once disappeared every 34 minutes, and the recent rate is one every 100. Upstream, the Old River Control Structure sends about 30% of the combined Mississippi–Red flow down the Atchafalaya, which is the river's second mouth on the map.
Why it matters · what to notice on the map
Read the map from the rim to the mouth
The rim is a ridge line. On the east side of the Continental Divide in Montana, rain heads for the Gulf. A few miles west, it heads for the Pacific.
The farms sit in the middle. The Corn Belt lies right where the tributaries converge, so its runoff has only one place to go.
The damage shows up far from the cause. The dead zone forms on the Louisiana–Texas shelf, roughly 900 miles from the Iowa fields that feed it.
There are really two mouths. The Atchafalaya carries roughly 30% of the flow. Engineers hold that split in place at Old River.