Learn Spanish (Latin America): Ayer había muchas libélulas cerca del río. - Yesterday there were many dragonflies near the river.

In Spanish, when haber is used impersonally to mean 'there is' or 'there are', it always stays in the singular form, even if the object following it is plural. So, while we say 'there were' in English, you must use the singular había for 'there were many dragonflies'. Using habían muchas libélulas is a common mistake even among some native speakers, but standard Spanish strictly requires the singular había.

The word ayer usually triggers the preterite tense, so why do we use the imperfect había instead of hubo?

When you want to set the scene or describe how things were in the background, you use the imperfect tense. Even though ayer gives a specific timeframe, you are describing the ongoing presence of the dragonflies at the river, not a single completed event. You would use the preterite hubo if a distinct event occurred, like ayer hubo un accidente (yesterday there was an accident).

Why do we use del in cerca del río?

The phrase for 'near' is cerca de. When the preposition de is immediately followed by the masculine singular article el (since the river is el río), they must contract into the single word del. You can never write cerca de el río.