San Francisco Bay behaves like several pieces of water that happen to share one entrance. Inside the bay, depth is mostly a question of where the ship channel runs and where the shoal starts, and the current decides how long any crossing actually takes. Go east past Benicia and the water narrows into rivers, with Collinsville, Antioch and Clarksburg strung along them. Go outside and turn north and the bottom becomes rock, past Bolinas Lagoon and Tomales Bay and on up to Arena Cove. Turn south instead and it does the same thing past Ano Nuevo Island toward Carmel Bay. A skipper who runs all of that in one season is reading three different kinds of chart.
The Helm pack for this region is built from 112 official NOAA ENC cells. It carries 51,821 individual charted soundings, 7,721 charted depth areas and 8,551 depth contours, plus 5,585 charted underwater rocks and 137 NOAA tide prediction stations. The whole download is 10 MB, which means the rivers, the bay and the outside coast are all on the phone before you leave the dock.
The largest single group of hazards charted in this region is rock. There are 5,585 charted underwater rocks in the pack, sixth highest of the 18 US regions Helm builds packs for, against 497 charted wrecks and 2,482 other obstructions. The ratio is the useful part. In a soft bottom region the thing that catches you is usually a shoal that has moved since the survey was run. Here it is usually something that has been sitting in exactly that spot since long before anybody surveyed it.
That works in your favor and against you at the same time. A charted rock does not migrate, so an old sounding over rock is still telling you something true today. But rock does not shelve either. Depth can go from comfortable to unusable inside a boat length, which is why the shoulder of a channel here is not the same kind of margin it would be on a sand coast, and why cutting inside a headland to save a quarter mile is a different sort of decision. Helm shades the water your draft cannot take and reports the shallowest point along any line you draw before you commit to it.
| In the chart pack | Count |
|---|---|
| NOAA ENC cells | 112 |
| Charted soundings | 51,821 |
| Depth areas | 7,721 |
| Depth contours | 8,551 |
| Lateral buoys | 249 |
| Lateral beacons | 939 |
| Charted lights | 1,437 |
| Anchorage areas | 121 |
| Bridges | 894 |
| Overhead cables | 543 |
| Dredged areas | 578 |
| Charted wrecks | 497 |
| Charted obstructions | 2,482 |
| Underwater rocks | 5,585 |
| Restricted areas | 541 |
| NOAA tide prediction stations | 137 |
Counted from the Helm chart pack for San Francisco & NorCal, built from official NOAA electronic navigational charts. The pack downloads at about 10 MB.
Read the charted depth across all 578 dredged areas in this pack and the median comes out at 10.3 meters, about 34 feet. The deepest runs 16.6 meters, near 54 feet. The shallowest is 0.3 meters, which is about a foot of water.
That spread is why the word dredged on its own is not information here. The same label sits on the deep water a ship uses on its way up the bay and on a cut into a slough that dries most of the way out at low water. Helm shows the charted depth for the area you are in rather than the category it belongs to, and says where that figure came from, so a channel survey and the chart on its own do not read as the same promise.
Most of the marked water in this region is marked with something bolted down. The pack holds 939 lateral beacons to 249 lateral buoys, along with 1,437 charted lights and 22 safe water marks, which puts the region sixth of the 18 for safe water. Up the sloughs and the river reaches the channel edge tends to be a pile with a dayboard on it rather than a can swinging on a chain.
A fixed mark is worth more than a floating one when the current is running, because it is where the chart says it is. A buoy sitting in a hard ebb leans and sets down current, and every bearing you take off it carries that error. The pack also carries 541 restricted areas, eighth of the 18 regions. That is a lot of boundary for one stretch of water, and it is the kind of line better drawn on the chart in front of you than recalled from memory halfway across.
Air draft counts for more here than in most of the water Helm covers. The pack charts 894 bridges and 543 overhead cables, and the cable count is eighth of the 18 regions. Cables cross the sloughs and the river reaches in places where the water underneath looks completely open, so the number that decides your afternoon is the one above the masthead rather than the one under the keel.
Helm reads the charted vertical clearance and checks it against the air draft you entered, including the down position of an opening bridge. The tide station list for this region includes the Brazos Drawbridge on the Napa River and the Borden Highway Bridge on Old River, which is a fair description of how the upper end of this water works. Up there the bridge and the tide are one problem, because the clearance you get is the clearance at the height of water you arrive on.
There are 137 NOAA tide prediction stations inside this pack. That density is not redundancy. The water at Alcatraz Island and the water at Collinsville share a tide in name only, and by the time you are up at Antioch or Clarksburg the timing has moved on you. A single station applied to the whole region would be wrong somewhere for most of the day, so Helm reads the station nearest the spot you are asking about and tells you which one it used.
A sense of the spread, using stations that are actually in the pack:
The same box holds 121 charted anchorage areas, fifth of the 18 US regions. Where you put the hook around here is a current decision before it is a wind decision, and having the anchorage boundaries on the same screen as the depth and the nearest tide station saves a fair amount of guessing at dusk.
These counts describe what the chart pack carries, not the state of the water today. Charted data ages, channels shoal and marks move. Keep a lookout and verify locally.
The one nearest the place you are asking about. There are 137 NOAA tide prediction stations inside this pack, from Arena Cove in the north down to Carmel Cove, and Helm names the station it used instead of applying one bay figure everywhere. Alcatraz Island, Benicia, Collinsville and Antioch are all in there as separate stations, which starts to matter as soon as you are east of the bay and the tide reaches you later than it reaches the islands.
Across the 578 dredged areas in this pack the median charted depth is 10.3 meters, about 34 feet. The deepest is 16.6 meters, near 54 feet, and the shallowest is 0.3 meters, roughly a foot. Dredged is a category here rather than a depth, so Helm shows the charted figure for the specific area you are in.
Both. The same 10 MB download reaches from Arena Cove on the north coast, east through Benicia to Collinsville, Antioch and Clarksburg, and south past Ano Nuevo Island to Carmel Bay. It is built from 112 NOAA ENC cells and carries the 894 bridges and 543 overhead cables that start to matter once you are in the river reaches, along with 121 charted anchorage areas.
Helm Boating