Filed by Crestwood Midstream Partners LP

pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: Crestwood Midstream Partners LP

Commission File No.: 001-33631

May 6, 2013

Dear Valued Customer,

I am writing to share with you some exciting news about our company. This morning we announced that Crestwood Holdings and Crestwood Midstream Partners LP will be combining with Inergy LP and Inergy Midstream Partners (NRGM) to create a fully integrated midstream partnership with a total enterprise value of more than $7 billion.

CMLP and NRGM are a natural fit. As you know, CMLP operates a first class portfolio of shale-focused midstream assets. Currently, our operational capabilities and services to our customers end at the tailgate of the processing plant, but with this combination we will have the size and scale to provide you with even better service across the midstream value chain, with a wider array of midstream services in more places. Inergy’s established business platform of providing oil, gas, and liquids transportation, marketing and logistics services is highly complementary to Crestwood’s gas gathering and processing and will enable the combined partnership to provide customers with a significantly broader range of capabilities to better meet their midstream needs going forward.

CMLP and NRGM share an established tradition of safe, efficient operations with a strong commitment to providing the best midstream services to our customers. We are excited to join with them in what we view as a merger of equals to continue serving you going forward.

It is important for you to know that until this multi-step transaction is fully completed, which we expect will occur in the third quarter of 2013, it remains business as usual at CMLP with the same leadership team overseeing operations. Following the close, we are confident that we will be able to provide you with enhanced value and services in the midstream process. In the meantime, your day-to-day contacts at the Company will stay the same and we remain committed to continuing to support you with the highest quality of service.

This is an exciting time for all of us. As always, if you have any questions, please feel free to reach out to your regular contact.

Sincerely,

Robert G. Phillips

Chairman, President and CEO

Additional Information and Where to Find It

This press release contains information about the proposed merger involving Crestwood and Inergy Midstream. In connection with the proposed merger, Inergy Midstream will file with the SEC a registration statement on Form S-4 that will include a proxy statement/prospectus for the unitholders of Crestwood. Crestwood will mail the final proxy statement/prospectus to its unitholders. INVESTORS AND UNITHOLDERS ARE URGED TO READ THE PROXY STATEMENT/PROSPECTUS AND OTHER RELEVANT DOCUMENTS FILED OR TO BE FILED WITH THE SEC CAREFULLY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT CRESTWOOD, INERGY MIDSTREAM, THE PROPOSED MERGER AND RELATED MATTERS. Investors and unitholders will be able to obtain free copies of the proxy statement/prospectus and other documents filed with the SEC by Inergy Midstream and Crestwood through the website maintained by the SEC at www.sec.gov. In addition, investors and unitholders will be able to obtain free copies of documents filed by Crestwood with the SEC from Crestwood’s website, www.crestwoodlp.com, under the heading “SEC Filings” in the “Investor Relations” tab and free copies of documents filed by Inergy Midstream with the SEC from Inergy Midstream’s website, www.inergylp.com/midstream, under the heading “SEC Filings” in the “Investor Relations” tab.

Participants in the Solicitation

Crestwood, Inergy Midstream, Inergy, L.P. and their respective general partner’s directors and executive officers may be deemed to be participants in the solicitation of proxies from the unitholders of Crestwood in respect of the proposed merger transaction. Information regarding the persons who may, under the rules of the SEC, be deemed participants in the solicitation of the unitholders of Crestwood in connection with the proposed transaction, including a description of their direct or indirect interests, by security holdings or otherwise, will be set forth in the proxy statement/prospectus when it is filed with the SEC. Information regarding Crestwood’s directors and executive officers is contained in Crestwood’s Annual Report on Form 10-K for the year ended December 31, 2012, which is filed with the SEC. Information regarding Inergy’s directors and executive officers is contained in Inergy Midstream’s Annual Report on Form 10-K for the year ended September 30, 2012, which is filed with the SEC. Information regarding Inergy’s directors and executive officers is contained in Inergy, L.P.’s Annual Report on Form 10-K for the year ended September 30, 2012, which is filed with the SEC. Free copies of these documents may be obtained from the sources described above.

Forward Looking Statements

The statements in this communication regarding future events, occurrences, circumstances, activities, performance, outcomes and results are forward-looking statements. Although these statements reflect the current views, assumptions and expectations of Crestwood’s and Inergy’s management, the matters addressed herein are subject to numerous risks and uncertainties which could cause actual activities, performance, outcomes and results to differ materially from those indicated. Such forward-looking statements include, but are not limited to, statements about the future financial and operating results, objectives, expectations and intentions and other statements that are not historical facts. Factors that could result in such differences or otherwise materially affect Crestwood’s or Inergy’s financial condition, results of operations and cash flows include, without limitation, failure to satisfy closing conditions with respect to the merger; the risks that the Crestwood and Inergy businesses will not be integrated successfully or may take longer than anticipated; the possibility that expected synergies will not be realized, or will not be realized within the expected timeframe; fluctuations in oil, natural gas and NGL prices; the extent and success of drilling efforts, as well as the extent and quality of natural gas volumes produced within proximity of our assets; failure or delays by our customers in achieving expected

production in their natural gas projects; competitive conditions in our industry and their impact on our ability to connect natural gas supplies to our gathering and processing assets or systems; actions or inactions taken or non-performance by third parties, including suppliers, contractors, operators, processors, transporters and customers; our ability to consummate acquisitions, successfully integrate the acquired businesses, realize any cost savings and other synergies from any acquisition; changes in the availability and cost of capital; operating hazards, natural disasters, weather-related delays, casualty losses and other matters beyond our control; timely receipt of necessary government approvals and permits, our ability to control the costs of construction, including costs of materials, labor and right-of-way and other factors that may impact our ability to complete projects within budget and on schedule; the effects of existing and future laws and governmental regulations, including environmental and climate change requirements; the effects of existing and future litigation; and risks related to our substantial indebtedness, as well as other factors disclosed in Crestwood’s and Inergy’s filings with the U.S. Securities and Exchange Commission. You should read Crestwood’s and Inergy’s filings with the U.S. Securities and Exchange Commission, including their Annual Report on Form 10-K for the year ended December 31, 2012, and September 30, 2012, respectively, and their most recent Quarterly Reports and Current Reports for a more extensive list of factors that could affect results. Neither Crestwood nor Inergy assumes any obligation to update these forward-looking statements.