Though it's not common knowledge today, there used to be a waterway that burbled through the heart of downtown San Francisco. This was Hayes Creek, a since-culverted gully that stretches under Hayes Valley and just south of City Hall.

The Oakland Museum of California gives a little insight into the disappeared stream:

This historical creek does not appear on most old maps of the city, so it was probably ephemeral, only flowing in wet weather. The rest of the year, it would have percolated underground. It is said that a high groundwater table can be found at the basement of the San Francisco Symphony Hall and other Civic Center buildings.

There was once a time when San Francisco was glistening with creeks and arroyos, or streams that stay dry for part of the year. When Spanish explorers arrived in what's now the Lower Haight in the late 1700s, they found a healthy brook and named it Fuente de Dolores. Down in the Mission there was a gulch whose water helped sustain cattle and crops. In 1878, the municipal government took another natural channel under modern-day Cesar Chavez Street and turned it into a sewer.

These hidden runnels are set to receive a high-profile tribute during April's Market Street Prototyping Festival. (Some of the other projects, including a fog generator, are shown here.) Artists will decorate the roadways around City Hall with snaking pathways approximating the location of the surface cricks of yore. The placemaking venture, "Ghost Arroyos," is a collaboration between designer Emily Schlickman and indie radio producer Kristina Loring. Here's a bit about it:

Every city has invisible histories embedded within its landscape. Up until the 19th century, ephemeral streams ran through nearly every valley in San Francisco, channeling rainwater to peripheral tidal estuaries. This project, "Ghost Arroyos" seeks to reveal these forgotten waterways of the city through a simple, but powerful intervention. Situated between 7th and 9th street, the project will mark the historical footprint of the arroyos onto the urban surface through paint or lighting. Visitors to the festival will be invited to trace the path of the waterways while listening to a curated recording of hydrological soundscapes and oral histories.

And what might these phantasmal streams look like? This image gives a tantalizing preview: