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The federal government is well into its third week of a partial shutdown, and President Donald Trump appears no closer to a deal with Democratic leaders to reopen several key agencies.

As a result, more and more government functions are faltering, and an estimated 800,000 federal employees are facing the prospect of missing their first paychecks of 2019.

With negotiations at a standstill, Trump has threatened to keep key agencies shuttered for months or even a year if Democrats don't agree to fund his desired border wall, and he is considering declaring a national emergency to try to get it done without them.

Here are some of the ways the country has already been affected by the shutdown:

National parks are filled with trash

National parks have remained largely open during the shutdown, but visitors have been left to their own devices with thousands of park employees furloughed. Trash removal and general maintenance have been put on hold, leaving parks to fill up with trash and bathrooms to overflow with human waste.

Volunteers have tried their best to keep the parks clean, spending hours removing litter and cleaning bathrooms, but they are fighting an uphill battle.

On Tuesday, the National Park Service announced that Joshua Tree National Park in California would temporarily close because unsupervised tourists had been damaging the park's land and its iconic Joshua trees.

The National Zoo and Smithsonian museums in Washington have also closed after initially remaining open until January 1. Even the zoo's beloved panda cam was not spared, going dark the morning after the zoo locked its gates.

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Uncollected trash sits on the National Mall in Washington on Dec. 31, 2018. Erik S. Lesser / EPA

Unpaid TSA workers are calling out sick

Transportation Security Administration employees have remained at work during the shutdown and are now set to miss their paychecks, which would normally be issued Friday. That has raised concerns that more TSA employees could call in sick to find alternative sources of income.

Last Friday, 5.5 percent of TSA employees at Dallas/Fort Worth International Airport called out sick -- two percent more than normal, according to an agency spokesman. If there's an increase in call-outs, it could lead to longer lines and increased wait times for passengers.

Some TSA employees are even taking the desperate step of quitting their jobs to find a guaranteed paycheck, Hydrick Thomas, president of the American Federation of Government Employees' TSA Council.

"Every day I'm getting calls from my members about their extreme financial hardships and need for a paycheck," Thomas said in a statement Tuesday. "Some of them have already quit and many are considering quitting the federal workforce because of this shutdown."

Coast Guard members continue to work without pay

About 42,000 active-duty members of the Coast Guard have continued to work without pay because, unlike other military personnel, their agency is part of the Department of Homeland Security, which has seen its funding lapse. The Department of Defense, in contrast, will remain fully funded through September 2019.

Most of the Coast Guard's civilian workforce has been furloughed, leaving the active-duty members without their full network of support staff. Because of those limitations, the Coast Guard is providing only essential operations that protect life and property or national security, a spokesperson told NBC News.

Low-income housing subsidies not being renewed

The Department of Housing and Urban Development has been especially hard hit by the shutdown -- 95 percent of HUD employees are furloughed. With a skeleton staff and no funding, HUD has been unable to renew more than 1,000 contracts that provide federal subsides to landlords who own and operate Section 8 housing. Hundreds more contracts could expire if the shutdown continues into February. The situation could delay critical repairs and place poor families at risk of eviction, advocates and landlords say.

HUD has advised landlords to use reserve funds to cover costs until the shutdown ends and contracts can be renewed. For many landlords who own low-income housing buildings, that means limited funds to deal with building maintenance and emergencies that may arise.

Food stamps at risk if the shutdown goes past February

The tens of millions of Americans on food stamps will receive their benefits for February, but that aid could be cut off if the shutdown continues into March.

Agriculture Secretary Sonny Perdue announced Tuesday that his department would issue February's benefits a little early to take advantage of a clause in the stopgap spending bill that expired in December that allows the government to distribute required payments within 30 days of the bill's expiration.

The food stamps program has only $3 billion in emergency reserves — not enough to pay for a full month's assistance — and because of the shutdown, the Department of Agriculture has no money to pay for the program in the ensuing months.