Voyage to the City of the Dead

"Many worlds of the Humanx Commonwealth boasted of 'natural wonders' but Horseye was truly unique -- the planet had the most spectacular river valley anywhere in the known universe and was home to three alien cultures. The fascinating planet just cried out for proper study, and after months of impatient quarantine Etienne and Lyra Redowl had finally received permission to begin a voyage of exploration to the source of the River Skar, a mere 12,000 kilometers Upriver. Old hands at cracking new planets, the Redowls studied the aliens languages, took local guides, and provided for emergencies. But nothing could prepare them for the awesome treachery of the natives or the unbelievable natural obstacles. And not even the natives understood the planet's deepest secret..." From Goodreads