Jisho

Firstly, in case you weren't aware, Jisho does not create any of its own entries. All of the entries come from other data sources.

This is an automatically-generated entry based on a Wikipedia article. Entries that don't come from the main dictionary sources don't have readings included.

Anyone can submit an entry to JMdict, and if it's accepted, then Jisho would use that data.

The reading usually is given in brackets after the word's first mention in the linked Wikipedia article.

Here:
三角江（さんかくこう）は、平野を流れる河川が沈水して出来た三角状の入り江[1]，あるいはその河口部である[1]。エスチュアリー[2]、河口湾ともいう[1]。