# EPOD - a service of USRA

## The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

### Knik Arm Tides of Alaska

November 18, 2021

Photographer: Gabrielle Tepp

Summary Author: Gabrielle Tepp

Oceans are well known to have twice daily tides, but not all tides are equal. Among many factors, the shape of bays, topology of the sea floor, and phase of the moon all affect the size of tides. Cook Inlet, particularly Turnagain Arm, has the third largest tidal range in North America and the largest in the United States. These photos show the difference in tides in the Knik Arm of Cook Inlet, along the coast of Anchorage, Alaska, where the average tidal range is about 26 ft (8 m). The top photo was taken on August 21, 2021 near the peak time of a 30 ft (9.2 m) high tide. In contrast, the bottom photo was taken three days later near a low tide of -1.5 ft (-0.5 m). The difference is quite dramatic! While hard to tell from the photos, the gully is deep enough for a person to fit in. Fish Creek is located to the right of the imaged area and typically flows into the gully with a small waterfall. The highest tides, however, fill the gully to the same level as the creek. When the tide is low, mudflats made of glacial silt are revealed. Shorebirds like to feed in the small pools of water that form on the mud. Be careful if you decide to wander out though – it’s easy to get stuck in the mud and you don’t want to be stranded out there when the tide comes in!

- Anchorage, Alaska Coordinates: 61.437, -149.770
- Related Links
  - Tides Near Port Orford, Oregon
- Student Links
  - Interactive Tide Map
  - More Information About Tides
- Earth Observatory
  - Anchorage and Cook Inlet, Alaska

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