Herbert Deakyne, Corps of Engineers, on preliminary examination and survey, respectively, of Pinole Shoal and Mare Island Channel and turning basin, Calif., authorized by the river and harbor act approved September 22, 1922.

2. Pinole Shoal occupies the southeastern portion of San Pablo Bay, and the channel through it is the main ship channel connecting San Francisco Bay proper with Carquinez Strait, Suisun Bay, and the Sacramento and San Joaquin Rivers. Mare Island Strait is the term applied to the lower improved stretch of Napa Creek, which enters Carquinez Strait just east of the point where the latter joins San Pablo Bay. The United States has improved these waterways under a. project for a channel 35 feet deep at mean lower low water and 500 feet wide across Pinole Shoal, and of the same depth and width through Mare Island Strait, with a 1,000-foot turning basin opposite the upper end of the navy yard. The latter improvement is known as Mare Island Channel. Range of tide between mean lower low water and mean higher high water is 6.5 feet. Local interests have made certain requests for work in San Pablo and Suisun Bays and in Carquinez Strait, which are outside the scope of the item of law authorizing the present investigation. The Navy Department states that its needs can be met by a channel 35 feet by 500 feet across Pinole Shoal and 30 feet by 600 feet in Mare Island Strait, with a slight increase in the area of the turning basin.

3. The district engineer points out that Pinole Shoal Channel carries a large and important commerce, amounting in 1923 to about eight and a quarter million tons, of which much is in ocean vessels drawing up to 30 feet or more. This can be adequately accommodated by the 35-foot depth requested by the Navy Department. In view, however, of the large amount of shipping, the difficulty in maintaining the channel at all times to full project depth and width, and the prevalence of fogs, he believes that the width should be increased to 600 feet. Mare Island Strait carries a considerable commercial traffic, all of which, however, can be accommodated by a channel of the dimensions requested by the Navy Department. These dimensions appear necessary in the interest of national defense.

4. The district engineer submits the results of a careful study of the silting which occurs in the two channels under consideration and of the means which might be adopted to reduce it. The study includes results of monthly and bimonthly surveys extending over about a year, and of borings, current observations, and studies of the character of the silt deposits. He concludes that the basic cause for the shoaling is the existence of immense deposits of soft material in San Pablo Bay, brought down in the past by the Sacramento and San Joaquin Rivers, the volume of which deposits was greatly increased by uncontrolled hydraulic mining operations. The silting in Mare Island Strait is caused largely by the action of the flood tide, which, advancing up San Francisco Bay, moves across the broad shallow expanse of water in the northern part of San Pablo Bay, picks up the light material there deposited and carries it into Carquinez Strait and up Mare Island Strait. The ebb tide removes some but not all of the material thus brought in. A certain amount is also carried down into Mare Island Strait from the tidal marshes along the upper reaches of Napa Creek.

5. Three possible methods are discussed for improving conditions in Mare Island Channel. The first is by a dam across Napa Creek above the navy yard, and the diversion of the creek and its burden of silt into San Pablo Bay. This would not, however, affect the major source of the deposits, which, as stated, is the silt from San Pablo Bay brought in by the flood tide. The diversion dam would also be quite expensive, would restrict the extension upstream of commercial developments on the opposite side of the strait from the navy yard, would interfere with the light-draft navigation now existing on the upper reaches of Napa Creek, and might have a deleterious effect on the regimen of Pinole Shoal Channel. The district engineer therefore does not consider the dam a sound solution.

6. The two other solutions are regulating works and dredging. The Navy Department has in the past provided dikes on, and at the entrance to, Mare Island Strait, which have had a beneficial effect on the channel. Some of these have deteriorated, but the Navy Department proposes repairs and modifications which in the opinion of the district engineer are adequate and satisfactory. There was also constructed, in 1912, a dike running southwest and west from the mouth of Mare Island Strait into San Pablo Bay, with a view to catching the silt brought across that bay by the flood tide. This dike served a useful purpose, but the amount of silt deposited behind it was so great that its utility as a catch basin is now very limited. The district engineer considers a possible extension of the dike or the construction of a supplementary dike north of and parallel to it. Due to the activity of marine borers, and to the pressure against such a dike on account of tidal head and silt deposits, it would be necessary to make the structure of treated timber and to reinforce it initially or eventually with riprap. An expenditure of several million dollars. would be required, the deposits behind the dike would reduce the tidal prism in San Pablo Bay, and it is unlikely that the saving in the cost of maintaining Mare Island Channel would be as great as the carrying and maintenance charges on the dike. The district engineer accordingly feels that no further regulating works are justified than those now in existence or proposed by the Navy Department.

7. The only remaining method of maintenance is dredging. This has been carried on on a large scale for some years by the department, which as a result of recent studies and experiments has been able to reduce the unit cost of the work and to maintain the channels more adequately with the funds available. The district engineer submits various estimates for the dredging and maintenance of channels of different dimensions. Those now proposed involve a first cost of $481,000, with annual maintenance of $135,000 for Pinole Shoal and $205,000 for Mare Island Channel and turning basin, a total maintenance cost of $340,000. He recommends adoption of such channels under these estimates.

8. These reports have been referred, as required by law, to the Board of Engineers for Rivers and Harbors, and attention is invited to its report herewith, agreeing with the district engineer.

9. After due consideration of the above-mentioned reports, I concur in the views of the district engineer and the Board of Engineers for Rivers and Harbors. Pinole Shoal Channel is of very great commercial importance. The 35-foot depth now provided has been amply justified by existing traffic, and the volume and diffi

culties of navigation make proper the 100-foot increase in the width now proposed, which can be provided for a small part of the total cost. The channel now requested by the Navy Department in Mare Island Strait is 100 feet wider than the existing project channel, but has a depth 5 feet less. Maintenance of such a channel is a national defense matter, and the dimensions proposed for this purpose by the Navy Department are adequate commercially. The principal question here is how most economically to accomplish the large amount of maintenance which is continuously necessary. The careful studies embodied in the district engineer's report are supplementary to, and in general confirm, the results of previous studies made by the War and Navy Departments. Diversion of Napa Creek would be an expensive and inadequate solution and possesses several other drawbacks. Regulation has been undertaken on a considerable scale, and it does not appear that further work of this character would in the long run. be economical. It is therefore necessary to continue the dredging work which is now being carried on, and which, by the investigations and experiments of the department, has been brought to a high degree of efficiency. The estimates of the district engineer for the work proposed appear reasonable and adequate. I therefore report that modification of the existing project for Pinole Shoal and Mare Island Channel and turning basin, Calif., is deemed advisable to the extent of providing a channel 35 feet deep and 600 feet wide across Pinole Shoal in San Pablo Bay, and thence a channel 30 feet deep and 600 feet wide in Mare Island Strait, with a turning basin of the same depth, except at the northeasterly corner where it should be 26 feet, and 1,000 feet wide, along the general lines proposed by the district engineer, at an estimated cost of $480,000, with $340,000 annually for maintenance. Funds for new work should be made available in three approximately equal annual installments.

H. TAYLOR,

Major General, Chief of Engineers.

REPORT OF THE BOARD OF ENGINEERS FOR RIVERS AND

HARBORS

SYLLABUS

The Board of Engineers for Rivers and Harbors concurs with the district engineer in recommending modification of the existing project for the improvement of San Pablo Bay and Mare Island Strait, so as to provide for a channel 35 feet deep and 600 feet wide across Pinole Shoal in San Pablo Bay, and thence a channel 30 feet deep and 600 feet wide in Mare Island Strait, with a turning basin of the same depth, except at the upper end where it should be 26 feet and 1,000 feet wide, at an estimated cost of $481,000, with $340,000 annually for maintenance

[Second indorsement]

BOARD OF ENGINEERS FOR RIVERS AND HARBORS,

Washington, D. C., September 29, 1925.

To the CHIEF OF ENGINEERS, UNITED STATES ARMY:

1. The following is in review of the reports on preliminary examination and survey of Pinole Shoal and Mare Island Channel and turning basin, Calif., authorized by the river and harbor act approved September 22, 1922.

2. San Pablo Bay is an extension of San Francisco Bay to the north. On the east it connects, via Carquinez Strait, with Suisun Bay and the mouths of the San Joaquin and Sacramento Rivers. Pinole Shoal is in the main channel which passes through the southeastern part of San Pablo Bay. The Napa River, a small stream largely tidal, flows south into Carquinez Strait, entering it at a point just east of its junction with San Pablo Bay. The lower section of Napa River is known as Mare Island Strait, on the west side of which is the Mare Island Navy Yard. The locality is under improvement under a project providing for dredging a channel across Pinole Shoal 44,000 feet long, 35 feet deep at mean lower low water, and 500 feet wide, and thence through Mare Island Strait a channel 16,000 feet long, 35 feet deep, and 500 feet wide, with a turning basin 1,000 feet wide at the upper end. Range of tide between mean lower low water and mean higher high water is 6.5 feet.

3. Local commercial interests are satisfied with maintenance of a channel of the present dimensions across Pinole Shoal. They have requested additional work in Carquinez Strait and Suisun Bay and in front of a terminal on the south shore of San Pablo Bay, which, however, can not be covered under the item of law directing the present investigation. The Secretary of the Navy has stated that the needs of the Navy Department can be met by a channel 35 feet by 500 feet over Pinole Shoals and 30 feet by 600 feet in Mare Island Strait.

4. The San Pablo and Mare Island Channels are the means of approach to the Mare Island Navy Yard, and as a national defense measure should be maintained at whatever dimensions are required by the Navy and are economically practicable. Commercially, Pinole Shoal Channel handles an important commerce, amounting in 1923 to 8,225,000 tons. Some of this was in medium and light draft vessels operating on the Suisun Bay channels, and on the San Joaquin and Sacramento Rivers. A large part, however, pertaining to terminals on San Pablo Bay, Carquinez Strait and Mare Island Strait, was handled by deep-draft vessels. The principal items of this latter commerce were petroleum products, sugar, grain, and lumber. The district engineer, who is also the division engineer, states that these items are carried in vessels usually loaded to about the following drafts: 30 feet for sugar, 28 feet for grain, 27 feet for fuel-oil tankers, and 18 feet for lumber schooners. The maximum draft which may be expected for the present is about 32 feet. In view of the magnitude of the commerce, the large vessels in operation, the difficulty of maintaining the channel at full project dimensions at all times, and the prevalence of fogs, which make navigation difficult and may require vessels to anchor in the channel, the district engineer believes that the Pinole Shoal Channel should, for commercial purposes, be maintained, if economically practicable, at a depth of 35 feet, the same as that requested by the Navy, and a width of 600 feet, 100 feet wider than the minimum stated by the Navy as necessary for its purposes. Mare Island Strait, in addition to its use by the Navy, handles a considerable amount of commercial traffic, totaling in 1923 about 325,000 tons, of which 100,000 were exports. The commercial needs here, however, will be adequately served by the channel desired by the Navy.

5. These channels present a difficult and expensive maintenance problem, which has been repeatedly investigated in the past by the War and Navy Departments. The district engineer, in connection with the present report, has supplemented former studies and brought them up to date by careful investigations involving monthly and bimonthly surveys over a period of more than a year, borings, current observations, and the analysis and classification of the material deposited and carried in suspension in the channels and connecting waters under varying conditions of wind and tide.

6. San Pablo Bay, like Suisun Bay above it, is a natural settling basin for sediment brought down by the San Joaquin and Sacramento Rivers. Sedimentation occurs not only in the comparatively deep water over Pinole Shoal, which is the natural channel, but over the large shoal area to the north and west of this channel. The normal rate of deposit in the bay has been greatly increased, in the period since 1849, by the effects of large-scale hydraulic mining in the Valley of California. It has been estimated by the Geological Survey that from 1849 to 1914 approximately 1,146,000,000 cubic yards of débris were carried into San Francisco Bay and its connections, of which 570,000,000 were deposited in San Pablo Bay. This immense deposit profoundly modified the previous existing regimen and accelerated the shoaling of all the waters affected.

7. The effect of the deposits over Pinole Shoal was to decrease the depth by several feet. This has since been compensated for by dredging. Continued maintenance of the channel is made necessary by deposits from the neighboring shoals, and from the silt-bearing waters of the Sacramento and San Joaquin which enter the bay through Carquinez Strait.

8. The regimen of Mare Island Strait has in the past 75 years been much more affected than that of the Pinole Shoal Channel. Soundings in the strait are of record as far back as 1851. While not all of the early ones are believed to be entirely accurate, there is clear evidence of a steady decrease in natural depths. In 1851, the controlling depth at the mouth was given as 342 feet, and the least depth in the channel as 27 feet, this existing over but a small area. Soundings dating from 1896 to 1898 show 21 feet over the bar and a minimum depth of 15 feet for the inner channel.

9. Previous reports have expressed the opinion that the deposits in Mare Island Straits are principally from San Pablo Bay. The flood tide entering this bay at the southern end sweeps north and northeast over the broad shallow flats in the northern part of the bay, and then turns east and southeast to enter Carquinez Strait. The mud deposits on the flats, being of very fine material, are picked up and carried by the flood tide. This action is greatly accelerated when favoring winds produce wave action which stirs up the mud. The tidal current, charged with sediment, enters Mare Island Strait and flows up the Napa River to the tidal marshes above, depositing material as it goes. On the ebb tide a reverse action takes place, but observations indicate that the deposit materially exceeds the scour. Deposits are also caused in Mare Island Strait by material brought down by the ebb tide from the Napa River marshes. It has been estimated very roughly that of the total deposits, perhaps 25 per cent are due to material brought down Napa River, and 75 per cent to material brought from San Pablo Bay.