For decades, Texas farmers Randy and Kathy Reavis have been blessed by the Rio Grande River.

Key points: Construction on the border wall between the US and Mexico could begin as early as April 2019

Construction on the border wall between the US and Mexico could begin as early as April 2019 The wall was one of US President Donald Trump's core election promises

The wall was one of US President Donald Trump's core election promises Farmers in Texas could see their properties cut in half, but some still support the plan

In lands often savaged by drought, the river — which is also the border with Mexico — is the life blood that allows Mr Reavis to boast about their crop.

"These are some of the sweetest onions grown in the US, and I'll put 'em up against any state," Mr Reavis said.

The couple celebrated the early years of their marriage waterskiing and enjoying beer by the water.

"I've lived here all my life except for a few years in Mexico. Met this guy and the rest is history," Mrs Reavis said.

But their picturesque life on the banks of the river is about to change forever.

Construction of the border wall along the US-Mexico border — a core election promise made by US President Donald Trump — is expected to begin in April.

Mr Trump declared a national emergency to fund the wall, now survey pegs are in place and construction could begin any time — right through the Reavis' farm.

The wall will be built about 1.5 kilometres back from the river, meaning many properties will be cut in half.

For farmers, much of the best land will be on the wrong side of the wall. But Randy and Kathy Reavis still back the President's plan.

Construction puts farmers' livelihoods at stake

The Reavises' 600-hectare onion-growing operation could be destroyed by the border wall. ( ABC News: John Mees )

The Reavises stand to see 600 hectares of prime farm land sectioned off, which they will have to access through a gate with a security code.

Access to their pumps is not guaranteed.

"The greatest impact is if they cut us off from our pumps and the water, the rest of this is worthless. This type of soil is river soil, and things won't grow without the water," Mr Reavis said.

"If we lose our access to water, farming in this area is over."

But the Reavises would rather abandon these fields than see the wall stopped.

"It's a great [area], it really is," Mrs Reavis said.

"But before we used to leave the doors open, you left your house unlocked. Everything was safe and secure. But now it's not so safe. We don't feel it's safe."

McAllen, the city nearest to their farm, is considered safe by American standards, and has seen crime decrease in recent years.

But the Rio Grande Valley has become the busiest place for illegal border crossings in the US.

In a single night, as many as 1,200 immigrants are arrested trying to reach Texas from Mexico. Officials suspect many more make it across the porous border.

Border patrol agents in southern Texas arrest up to 1,200 immigrants crossing the border in a night. ( ABC News: John Mees )

Mr Reavis feels under siege.

"Personally, I think we're being invaded by a foreign country whose people are coming here, don't really give a damn about our culture, our rules, our law, our language. It's just not right," he said.

Mr Trump uses similar language to describe immigrants.

Loading

Currently, immigration centres are full, so the US has returned to a catch-and-release policy where people crossing illegally are released and must return for a later court date.

Justice Department data suggests approximately 90 per cent of those migrants appear before the court.

"Nobody really wants a wall, but times have changed. When you get people coming across in groups of thousands, when they hit, there's nothing to stop them," Mrs Reavis said.

Most of the groups crossing the border are families.

US Immigration and Customs Enforcement claims they find criminal history in two-thirds of detainees in the US, though some accuse them of inflating the statistics by including minor charges such as traffic violations.

Is it really worth losing the farm over?

"Nobody really wants a wall, but times have changed," Mrs Reavis said. ( ABC News: John Mees )

Kathy said for her, it is.

She added that the fear over security is so strong that many residents along the Rio Grand have started arming themselves.

"I know one woman who is now carrying a shotgun in her vehicle. She lives along the river. She's just had it up to here," she said.

Others see the wall as a nuisance

Unlike the Reavises, some land owners are planning to fight wall construction through the courts.

Just a few miles further along the river is the National Butterfly Centre, a 40-hectare nature preserve.

The centre's director Mariana Wright is deeply opposed to the wall.

"We have become the piñata for this administration. They've strung us up, and they're just beating the border lands with a stick, and of course they have the added benefit of targeting people of colour," she said.

The wall will leave 70 per cent of the nature reserve on the wrong side of the wall, and Ms Wright said animals would be trapped.

"The wall itself is going to be [5.4 metre high] concrete, with [5.4 metre tall] concrete-filled steel bollards on top of that. Then they're going to put up these lights with the all-night blitzkrieg lighting," Ms Wright said.

From her perspective, it's even more infuriating that the wall will have gates with access codes; she believes that will make the barrier completely ineffective.

"The bad hombres, they'll have the gate codes… anybody can go through the border wall anytime day or night as long as they have the code, and it'll be easy to obtain," she said.