Death on the River Walk, by Carolyn Hart (1999)

Carolyn Hart's heroine in this series of mysteries is Henrie O, doubtless a takeoff on the famous author of short stories with a twist, O. Henry.

Death on the River Walk, by Carolyn Hart (1999)

Backmatter:
Pulitzer Prize-winning ex-reporter Henrietta O'Dwyer Collins no longer chases hot stories all over the world, but murderous mysteries seem to find her. This time, a frantic phone call from an old and dear friend on the other side of the world sends Henrie O rushing to the fabled city of San Antion, (Texas) to check out the baffling disappearance of her friend's devoted granddaughter, Iris Chavez. Iris, employed at the Tesoros Gallery on SAn Antonio's famous River Walk, has suddenly dropped from sight without a word.

Soon Henrie O discovers that amidst the exquisite objects in the prestigious gallery and among the family members is hidden a dark secret--one Henrie O must uncover if she is to find Iris. Late one dark night on the River Walk, Henrie O sees a sprawled body...and realizes that treachery and disgrace lurk in the shadows of an old and respected business...and death awaits anyone daring to uncover the truth.

Opening paragraph

I glanced at the computer printout that rested on the passenger seat of the rental car, a casual picture of a grandmother and granddaughter, arms linked, faces aglow with laughter and love. The bright photograph had been scanned into a computer half a world awayand the resulting crisp picture that had issued from my daughter's computer was one of the small miracles that no one remarks in today's technological wonderland. The grandmother, Gina Wilson, was one of my oldest friends, a shining memory from the happiest years of my life. The granddaughter, Iris Chavez, was a child I'd come to know because she spent so much of her growing up time with Gina. Iris was near in age to my own granddaughter, Diana.

From Wikipedia:
Hart's Henrie O mysteries feature 70-something retired newswoman, Henrie O'Dwyer Collins, as she travels the country and the world, solving crimes that seem to follow her as she travels. Henrie gets by on her grit, tenacity, and sensible shoes.

Dead Man's Island (1993)
Scandal in Fair Haven (1994)
Death in Lovers' Lane (1997)
Death in Paradise (1998)
Death on the River Walk (1999)
Resort to Murder (2001)
Set Sail for Murder (2007)

Liberty Falling, by Nevada Barr (1999)

Liberty Falling, by Nevada Barr, was published in 1999.

Each of Barr's books takes place in some national park... on this occasion, the park is the Ellis Island National Monument - the island on which stands the Statue of Liberty, and the buildings that greeted legal immigrants during the 1920s.

Anna's sister, Molly, is deathly ill in a New York hospital, and Anna Pigeon has come to be with her, and in her spare time, to keep her sanity, she explores Liberty Island, and, as usual, comes across murder...and this time, a plot that could change America forever.

Here's the list of Nevada Barr books and the National Parks in which they take place:

Track of the Cat --Guadalupe Mountains National Park --1993
A Superior Death --Isle Royale National Park --1994
Ill Wind --Mesa Verde --National Park --1995
Firestorm --Lassen Volcanic National Park --1996
Endangered Species --Cumberland Island National Seashore --1997
Blind Descent --Carlsbad Caverns National Park --1998
Liberty Falling --Statue of Liberty National Monument --1999
Deep South --Natchez Trace Parkway --2000
Blood Lure -- Glacier National Park --2001
Hunting Season --Natchez Trace Parkway --February 2002
Flashback --Dry Tortugas National Park --February 2003
High Country --Yosemite National Park --February 2004
Hard Truth --Rocky Mountain National Park --March 2005
Winter Study --Isle Royale National Park --April 2008
Borderline --Big Bend National Park --April 2009
Burn New Orleans --Jazz National Historical Park August 2010