Johnston dam
Johnston
Johnston is a private gravity dam located in Half Moon Bay, California, built in 1919 for water supply purposes on the Arroyo Leon. With a height of 31 feet and a storage capacity of 30 acre-feet, Johnston serves as a vital water resource in the region. The dam has a low hazard potential and was last inspected in September 2020, with a satisfactory condition assessment reported in September 2017.
Managed by the California Department of Water Resources, Johnston is regulated, permitted, inspected, and enforced by state agencies to ensure its safety and compliance with regulations. The dam's primary purpose is water supply, with no associated structures and no federal involvement in its ownership, funding, design, construction, or operations. Despite its age, Johnston continues to play a crucial role in water management in San Mateo County, highlighting the importance of maintaining and monitoring aging infrastructure for climate resilience and sustainable water resource management.
With its historical significance and continued functionality, Johnston stands as a testament to the engineering and management of water resources in California. As climate change impacts water availability and quality, dams like Johnston play a key role in ensuring a reliable water supply for communities while also serving as potential tools for flood control and ecosystem management. As stakeholders continue to assess and address the risks associated with aging infrastructure, Johnston remains a valuable asset in the region's water infrastructure network.
Dam data reference
Condition Assessment
- Satisfactory
- No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
- Fair
- No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action.
- Poor
- A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency.
- Unsatisfactory
- A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
- Not Rated
- The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Hazard Potential Classification
- High
- Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
- Significant
- Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environmental damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
- Low
- Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
- Undetermined
- Dams for which a downstream hazard potential has not been designated or is not provided.
Plan around the weather
Same NOAA / yr.no feed Snoflo's iOS app uses. Watch the precipitation column on the meteogram -- rain on the basin upstream typically lifts inflow 24-72 hours later.
Next 5 days, hour by hour
Temperature line with weather symbols on top, snow + rain accumulation as columns, humidity as a dotted line.
5-day forecast table
Every 3 hours, broken out across temperature, snow, rain, humidity, and wind. Each cell is colour-coded relative to the column min/max.
| Time | Condition | Temp (°F) | Snow (in) | Rain (in) | Humidity (%) | Wind (mps) | Wind dir |
|---|---|---|---|---|---|---|---|
| Loading detailed forecast… | |||||||
15-day temperature & precipitation
Daily temperatures, snow, and rain projected over the next two weeks.
Nearby streamflow gauges
USGS streamgauges around Johnston -- inflows here typically show up in storage 24-72 hours later.
| Streamgauge | Discharge | View |
|---|---|---|
| Pilarcitos C A Half Moon Bay Ca | 10 cfs | → |
| Pilarcitos C Bl Stone Dam Nr Hillsborough Ca | 4 cfs | → |
| San Mateo C Bl Lo Crystal Sp Res Nr San Mateo Ca | 4 cfs | → |
| San Gregorio C A San Gregorio Ca | 14 cfs | → |
| San Francisquito C A Stanford University Ca | 6 cfs | → |
| Pescadero C Nr Pescadero Ca | 17 cfs | → |
Make a day of it
Boat launches, lakeside camping, fishing access, and other reservoirs near Johnston.
Boat launches
- Sawyer Camp Trail San Mateo County
- San Andreas Trail San Bruno
- J Hart Clinton Drive San Mateo
- Whidbey Lane Redwood City
- San Francisco Bay Trail, South San Francisco
- San Francisco Bay Trail East Palo Alto
Campgrounds
- Half Moon Bay State Beach
- Sweetwood Group Camp
- Dispersed Camping
- San Mateo Memorial County Park
- Camp Jones Gulch Ymca
- Shaw Flat Trail Camp (Primitive)
Fishing spots
Paddle runs
- Headwaters To Mouth
- Gold Creek To Three Pools (Opal Creek Run)
- Headwaters To Ends One Quarter Mile South Of Bear Valley Road Bridge Segment 1: Ends 1/4 Mile South Of Bear Valley Road Bridge Segment 2: Caltrans Corporation Yard At Point Reyes Staton.
- Olema Ranch Campground To Caltrans Corporation Yard At Point Reyes Station
More reservoirs
Track Johnston in the Snoflo app
Save this dam as a favorite and get the local NOAA / yr.no forecast plus regional flow context wherever you are.
About Johnston
Where does the data for Johnston come from?
Structural and regulatory data come from the U.S. Army Corps of Engineers' National Inventory of Dams (NID). Weather forecast comes from NOAA / yr.no -- the same feed Snoflo's iOS app uses.
How often is the report updated?
NID structural data refreshes annually as the Corps publishes updated assessments. The weather forecast refreshes throughout the day.
What does the Low hazard rating mean?
The Corps of Engineers' hazard potential classification grades probable consequences if the dam fails: High = probable loss of human life; Significant = no probable loss of human life but possible economic loss / environmental damage; Low = no probable loss of human life, only minor economic / environmental losses. See the Dam Data Reference card above for the full definitions.
What's "% of normal"?
The current storage value compared to the historical average storage on this calendar day. 100% = right on average; values above 100% mean above-normal storage (wet year); values below mean below-normal (dry year or drought).
Can I get alerts when storage crosses a threshold?
Yes -- alerts are managed in the Snoflo iOS app. Favorite this dam, set a threshold, and you'll get a push the moment conditions cross.
Other water bodies near here
Snoflo-tracked reservoirs and dams within driving distance of Johnston.