This article was updated on Sept. 6.

As the floodwaters recede, the full extent of Harvey’s toll is only beginning to emerge. Gov. Greg Abbott of Texas said the recovery costs might exceed those of Hurricane Katrina, which caused more than $100 billion in damage.

If you’re outside the affected area, here are options to help. (If you’re in Texas and displaced by the storm, here’s how to get help.)

Local organizations

The Hurricane Harvey Relief Fund of Houston’s mayor, Sylvester Turner, which is administered by the Greater Houston Community Foundation.

If you live in Texas, the City of Houston Emergency Operations Center has posted a list of places where you can take donations. In Dallas, officials have opened two donation centers. Additional details can be found here.

The Boys & Girls Clubs of Greater Houston is opening its doors at no charge this week, and offering extended hours. To donate, visit its website.

Houston Food Bank and the Food Bank of Corpus Christi are asking for donations.

The South Texas Blood and Tissue Center has posted an updated list of donor locations. Call 210-731-5590 or visit its website for more information.

Carter BloodCare covers hospitals in North, Central and East Texas. To donate, call 877-571-1000 or text DONATE4LIFE to 444-999.

To help animals, visit the Houston Humane Society or the San Antonio Humane Society. The Houston Society for the Prevention of Cruelty to Animals has set up an animal emergency response hotline (713-861-3010) and is accepting donations on its website.

The Texas Diaper Bank in San Antonio is asking for diapers and wipes, which can be dropped off in person or mailed to 5415 Bandera Road, Suite 504, San Antonio, Tex., 78238.

The United Way of Greater Houston flood relief fund will be used to help with immediate needs as well as long-term services like minor home repair. Visit its website to donate or text UWFLOOD to 41444.

The L.G.B.T.Q. Disaster Relief Fund will be used to help people “rebuild their lives through counseling, case management, direct assistance with shelf stable food, furniture, housing and more.”

The Federal Emergency Management Agency recommends checking with the National Voluntary Organizations Active in Disaster for a list of trusted disaster-relief organizations in Texas.

National organizations

All Hands Volunteers will be working on damaged homes and removing debris.

The American Kidney Fund is providing emergency financial grants to dialysis patients.

The American Red Cross is accepting donations. You can also text HARVEY to 90999 to donate $10.

AmeriCares takes medicine and supplies to survivors.

Catholic Charities provides food, clothing, shelter and support services to those from all religious backgrounds.

Direct Relief is shipping medicine and medical supplies to Texas, and has made its medical inventory of more than $100 million available for the Harvey relief effort.

Matthew 25: Ministries is distributing personal care kits, cleaning products, first aid and safety kits, diapers, paper products and tarps.

Donations to the Salvation Army can be made online, by calling 1-800-SAL-ARMY (1-800-725-2769) or texting STORM to 51555.

Save the Children is delivering baby supplies, including cribs and strollers, and setting up child-friendly spaces in shelters.

Samaritan’s Purse volunteer teams are helping homeowners by cutting trees and adding tarps to roofs, among other tasks.

SBP, a national disaster recovery nonprofit, will conduct damage assessment and help residents rebuild.

AABB, which coordinates a task force to manage blood collection during disasters, put out a call for blood donations. Most in demand: those with type O-positive blood.

Those interested in donating blood may contact the following organizations: AABB, America’s Blood Centers, American Red Cross and Armed Services Blood Program.

Online organizations

Airbnb is waiving service fees for those affected by the disaster and checking in between Aug. 23 and Sept. 25, and can guide users in creating a listing where their home is offered to victims free.

GoFundMe has created a page with all of its Harvey-related campaigns, including one started by the country singer Chris Young, who donated $100,000, and another created by the president and chief executive of the Rockport-Fulton Chamber of Commerce.

YouCaring has a fund-raising page set up by J. J. Watt of the Houston Texans with a goal of $20 million. On Tuesday evening, Mr. Watt posted a video to Twitter, announcing that the fund had reached its goal and thanking those who donated.

GlobalGiving’s Hurricane Harvey Relief Fund supports local organizations by helping to “meet survivors’ immediate needs for food, fuel, clean water, hygiene products and shelter.”

How to avoid being scammed

Before giving money to an organization, do your research.

Charity Navigator, which identifies worthy charities, has a list of organizations responding after the storm. Its database is a good starting place to research nonprofits.

The Internal Revenue Service has search tools that reveal whether an organization is eligible to receive tax-deductible charitable contributions.

If you suspect an organization or individual is engaging in fraud, you can report it to the National Center for Disaster Fraud.

For advice on avoiding fraudsters, read Charity Navigator’s post on how to protect yourself, and check out these tips from the Federal Trade Commission.

“Be wary of charities that spring up too suddenly in response to current events and natural disasters,” the F.T.C. website says. “Even if they are legitimate, they probably don’t have the infrastructure to get the donations to the affected area or people.”