Toxic Algae Bloom at Mission Trails Dam Site

Source: San Diego River Park Foundation

August 28, 2025 (San Diego) – Water samples collected last week at Old Mission Dam in Mission Trails Regional Park have tested positive for a cyanotoxin that can be harmful to people and pets.

The San Diego River Foundation has posted caution signs, in cooperation with the California Water Boards San Diego Region and rangers from the park owned by the city of San Diego.

This was detected at the lowest tier of the three tiered levels, but visitors should:

* avoid touching algae/scum

* do not drink the water

* do not let pets enter or drink the water

* keep children away from algae in the water and shore

* do not eat shellfish from the water

Algae blooms can occur naturally or from  urban runoff containing nitrogen in fertilizer. Not all are dangerous, but some form cyanotoxins which can be harmful or even deadly to people or pets.

You can help by avoiding use of fertilizers and by preventing water from leaving your yard.

For more information, visit https://www.sandiegoriver.org.